Gemara

(Beraisa): If one was Makdish a Ba'al Mum Kavu'a for the Mizbe'ach and it gave birth, the child may be sold without a Mum. It does not get Kedushas Guf, because what is Tefel (secondary) should not be greater than the Ikar (what is primary, i.e. the mother).

Shabbos 8b: Rebbi holds that Shevus (Isurim mid'Rabanan of Shabbos) do not apply Bein ha'Shemashos.

35b (Beraisa - Tana d'Vei R. Yishmael): We blow six shofar blasts on Erev Shabbos:

After the first, people in the field cease Melachah. After the second, stores are closed. After the third, pots are taken off the oven, and we are Matmin food (wrap it, to keep it warm) for the morning meal;

We blow six shofar blasts on Erev Shabbos to stop people from doing Melachah:

The first tells people in the field to cease Melachah. The second tells people in cities and stores to cease Melachah. The third tells people to light Ner Shabbos;

Rebbi says, the third tells people to remove Tefilin.

After the time needed to roast a fish or stick a dough to (a wall of) the oven, we blow three more sounds, and then everyone stops all Melachah.

Rishonim

Rambam (Hilchos Shabbos 4:4): When it is a Safek whether it is dark, one may wrap something hot.

Rambam (5:18): In all cities of Yisrael we blow six Teki'os on Erev Shabbos.

Rambam (19): After the third Teki'ah, people remove pots from the fire and wrap them and light Neros Shabbos.

Rambam (24:10): Everything forbidden due to Shevus (mid'Rabanan) was not decreed Bein ha'Shemashos, only during Shabbos proper. It is permitted Bein ha'Shemashos if needed for a Mitzvah or in pressed circumstances.

Poskim

Shulchan Aruch (OC 261:1): When it is Safek dark, i.e. Bein ha'Shemashos, one may not make Eruv Techumim. One may make Eruv Chatzeros.

Mishnah Berurah (7): Eruv Techumim are more stringent, for there is a support from a verse. Therefore, it is forbidden even for the sake of a Mitzvah. The Levush is lenient.

Kaf ha'Chayim (9): The Kesef Mishneh says that Eruv Chatzeros is not a Mitzvah. It is permitted Bein ha'Shemashos only if he was distracted and was unable to do it during the day. Aruch ha'Shulchan says that Eruv Chatzeros is a Mitzvah, and it is permitted Bein ha'Shemashos even if he was not distracted.

Shulchan Aruch (4): After answering Borchu, even if it is still day, one may not be Me'arev or wrap, for he accepted Shabbos on himself. For us, saying Mizmor Shir l'Yom ha'Shabbos is like Borchu for them.

Mishnah Berurah (28): Answering Borchu is like explicit acceptance of Shabbos. He is forbidden all the fences that Chazal made for Shabbos, e.g. entering a bathhouse merely to sweat, and all the more so to bathe. The Mechaber teaches that even those things permitted Bein ha'Shemashos are forbidden now. Since he explicitly accepted Shabbos long before Bein ha'Shemashos, this is more stringent than Bein ha'Shemashos without acceptance. We must say that the Heterim for Eruv Chatzeros and Hatmanah (wrapping to keep warm) Bein ha'Shemashos is where there is no Tzibur. Where there is a Tzibur, surely the majority accepted before Bein ha'Shemashos, and the minority is drawn after them. Many Acharonim say that this is only after Borchu, which is acceptance of the Tzibur. If an individual accepted Shabbos during the day, it is no more stringent than Bein ha'Shemashos. The Magen Avraham (7) connotes like this.

Shulchan Aruch (393:2): One may make Eruv Chatzeros Bein ha'Shemashos, even if he already accepted Shabbos. Some forbid of he accepted Tosefes Shabbos.

Chacham Tzvi (11): The Beis Yosef rules like R. Shmarayah, who initially permitted to be Me'arev after praying Ma'ariv, but retracted. A Tosefta says that at the Teki'os (sounding of the Shofar), even if he is holding a kettle, he must put it down (he may not wrap it to stay warm). This shows that acceptance is more stringent than Safek dark. That is difficult. One willingly accepts Tosefes Shabbos; it is a mere Mitzvas Aseh. How can it be more stringent than Bein ha'Shemashos, which is a Safek Isur Skilah?! If one did Melachah Bein ha'Shemashos before and after Shabbos, he brings a Chatas (for surely, he transgressed)! Rather, mid'Rabanan laws do not apply even after praying Ma'ariv. The Rambam holds like R. Yoel, who permits wrapping after the Teki'os. He permits every Shevus Bein ha'Shemashos; one may wrap something hot when it is Safek dark. He must hold that the Halachah does not follow the Tosefta (that forbids).

Machazik Berachah (261:5 DH va'Tabet): Chacham Tzvi did not understand the stringent opinion, how acceptance is more stringent than Safek dark. It is not difficult. Chachamim did not decree Bein ha'Shemashos. If one accepted, he accepted all laws of Shabbos, including Shevus. Indeed, if one did Melachah when it was Safek dark, it is a Safek Isur Skilah. Melachah after acceptance is more lenient. However, regarding whether one may be Me'arev, after acceptance is more stringent.

The Chacham Tzvi derived that the Rambam holds like R. Yoel. I say that this is because the Rambam holds that Tosefes Shabbos is not mid'Oraisa, like the Magid Mishneh (Shevisas Asor 1:6) says. The Beis Yosef in this Siman says that the Rambam holds that it is not even an Aseh mid'Rabanan. He holds that the Teki'os were not for Tosefes Shabbos, rather, so people will not do Isur Bein ha'Shemashos. Since we hold that Tosefes is mid'Oraisa, we can say that one who accepted is forbidden (even Shevus).

Mishnah Berurah (11): The stringent opinion forbids if he accepted, even if it is before Bein ha'Shemashos. This is more stringent than Bein ha'Shemashos, which comes by itself.

Kaf ha'Chayim (12): Eliyahu Rabah, Machazik Berachah and the Graz say that in Siman 261 we are stringent because the Tzibur accepted through saying Borchu. Here, only an individual accepted. If one prayed Ma'ariv, or even if he did not but the majority of the Tzibur prayed, all laws of Shabbos apply to him.

Rav Pe'alim (OC 49): We permit Shevus for the sake of a Mitzvah, or in pressed circumstances, when it is Safek dark without acceptance. If one accepted, R. Shmarayah forbids, and R. Yoel permits. The Mechaber (261:4) rules like R. Shmarayah, and did not even mention R. Yoel. In Siman 393, regarding Eruv Chatzeros, the Mechaber brought R. Yoel Stam, and added 'some say (like R. Shmarayah).' Eshel Avraham says that perhaps here (Siman 261) refers to Eruv Techumim. Eruv Chatzeros is mere Bitul Reshus (waiving his private ownership); it is more lenient. Eliyahu Rabah says here we discuss a Tzibur that accepted, and there we are lenient about an individual who accepted. Ma'amar Mordechai says that once the Mechaber revealed here that he is stringent, he did not need to be so precise in Siman 393, for we know how he rules. Dagul me'Revavah says that there we discuss one who accepted Tosefes Shabbos, which is only an Isur Aseh, so one may be lenient about Shevus. Here he discusses after answering Borchu, which is a true acceptance of Kedushas Shabbos. Therefore, he did not mention the lenient opinion at all. The Machazik Berachah and Maharikash answered well the Chacham Tzvi's objection.

Chasam Sofer (1:172 DH ul'Inyan): If a child became Bar Mitzvah on Motza'ei Yom Kipur, he was exempt from fasting. Likewise, he is exempt from adding to the fast at the end of the day even if he brought two hairs. The Tosefes is not part of the fast itself, to say that it extends into the night. There is no Kares or lashes for one who eats then. It is a mere Isur that Chazal expounded to add from Chol to Kodesh. If one was not obligated to treat the day like Kodesh, he is exempt from the Tosefes. On what can he add? There is no addition without the principal. This is unlike a Choleh who ate throughout on Yom Kipur, and he became healthy at the end of the day. Tosefes applies to him. The day was Kodesh for him, just he was permitted to eat due to Piku'ach Nefesh. He conducted with Kedushah, like Hash-m commanded, and now he adds to it. The minor did not treat it like Kodesh mid'Oraisa, so he has no Tosefes mid'Oraisa.

However, if a minor became Bar Mitzvah in Motza'ei Shevi'is, he may not guard (i.e. treat without Kedushah) Peros that grew during Shevi'is and are still attached in Sheminis. There is great reason to distinguish. There, the Kedushah was on the land and the Peros. They received Kedushas Shevi'is, and from their Chol (Sheminis) we add to the primary Kedushah. They are forbidden also to the minor after he matures. Yom Kipur is different. Its time passed. There is a Mitzvah on people to add to the Kodesh. Since the minor had no Kodesh, he has nothing to add. .

Rebuttal (Mishpetei Uzi'el OC 22): Once he matures, he is commanded to observe all the Mitzvos, even if they are continuations of Mitzvos that began earlier.

Question: If one forgot to count the Omer one day, he need not count at all afterwards, for we require "Temimos"! According to R. Eliezer, one who did not dwell in a Sukah on the first day does not build a Sukah during Chol ha'Mo'ed, for he cannot fulfill "ba'Sukos Teshvu Shiv'as Yamim"!

Answer: Sefiras ha'Omer is different. If he does not count all seven weeks, he does not fulfill the Mitzvah. The same applies to Sukah according to R. Eliezer. He holds that "ha'Ezrach" teaches that if one converted or matured during Sukos, he builds a Sukah. Why are they better than an adult Yisrael? The verse shows that the Torah was particular about a Yisrael who had a Chiyuv to be in a Sukah. The minor or convert were commanded only from when they matured or converted. The same applies to all Mitzvos that continue over time. They take effect on each person from when he became obligated to observe Mitzvos, even if he was exempt when the Mitzvah started.

Question: If he is liable only in the Tosefes, the Tefel (Tosefes, which is secondary) is greater than the Ikar (fast itself, which is primary)!

Answer (Mishpetei Uzi'el): That rule applies only when learning a law from a source, or when the primary Mitzvah applies in one place, and the Tefel is as great (elsewhere) as the Ikar, or anything that comes from the strength of another matter. These do not apply to Tosefes Yom Kipur. It is an additional Mitzvah to add time. Surely, it is logical that the extension is not more stringent than the primary time. To say that the minor was exempt on Yom Kipur, and now he is obligated (in this and all Mitzvos), is not a stringency over the primary time.

Va'Ya'an Yosef (OC 107): May a woman take an aspirin or pill after lighting the Neros? Even if she drinks water to swallow it, she need not bless, so this is not considered eating before Kidush. However, Refu'ah is forbidden (unless the entire body is weak) on Shabbos. It is permitted Bein ha'Shemashos, but she accepted Tosefes Shabbos, which is mid'Oraisa. Perhaps it is more stringent than Bein ha'Shemashos, which is only a Safek. Or, since it is only an Aseh, it is more lenient. Perhaps we can learn from the Rashba and all those who hold that the Torah forbids Sefekos, and even so Chachamim did not decree Bein ha'Shemashos. They decreed only when there is a Lav. Likewise, they did not decree about during Tosefes Shabbos. Also, the Torah forbids Melachah (due to Safek) during Bein ha'Shemashos, so Tosefes Shabbos must be beforehand. Even so, Shevus does not apply Bein ha'Shemashos (which is after Tosefes Shabbos). However, the Rambam could hold that one fulfills Tosefes Shabbos through refraining from Melachah at the end of Bein ha'Shemashos, so there is no proof according to him. If so, the answer to our question depends on these two opinions.