44a (Mishnah): Pigul applies to an Olah, for the blood permits the limbs to the Mizbe'ach. It applies to Parim and Se'irim ha'Nisrafim, for their blood permits the Eimurim to the Mizbe'ach.

83b (Ula): If Eimurim of Kodshim Kalim Alu before Zerikah, Lo Yerdu, for they became 'food' of the Mizbe'ach.

Yevamos 101a (Mishnah): From the knee and below, the Chalitzah is valid. From the knee and above, the Chalitzah is Pasul.

102a (R. Yanai): Whether he untied the shoe and she removed it, or she untied the shoe and he removed it, the Chalitzah is invalid. She must untie the shoe and remove it.

(Ameimar): The Yavam must press his foot on the ground.

(Ameimar): If one's foot is reversed, he may not do Chalitzah.

104b - Rabanan: If a Yevamah spat (at her Yavam), she must do Chalitzah.

Inference: Spitting disqualifies her from doing Yibum.

This is unlike R. Akiva. It is plausible to compare spitting to Eimurim. When the Eimurim are not around, offering them is not Me'akev (requisite for) eating the meat, but when they are around, they are Me'akev (one may not eat the meat before offering them). Even so, R. Akiva does not equate them. He says that failure to spit does not invalidate Chalitzah (even though it is possible).

Surely, he holds that spitting alone does not forbid Yibum!

This is unlike R. Eliezer. He holds that two things permit her (removing the shoe and spitting). In such a case, one does not permit without the other! Rather, it is like Rebbi;

Beraisa - Rebbi): The two loaves brought on Shavu'os become Kodesh only through Shechitah of the lambs offered with them;

If Shechitah and Zerikah of the lambs were done Lishman (with intent for the Mitzvah), the loaves become Kodesh. If Shechitah was Lo Lishmah and Zerikah was Lishman, the loaves are not Kodesh. If Shechitah was Lishmah but Zerikah was not Lishman, the loaves are Kodesh and not Kodesh;

R. Elazar b'Rebbi Shimon says, they are Kodesh only if Shechitah and Zerikah were Lishman.

Rif (Yevamos 33b): The Mishnah discusses when the shoe was tied above and below the knee. This teaches that his Shok (usually, this refers to the thigh. Here it refers to the lower leg) is called me'Al Raglo. Since she untied it, this is called Chalitzah "me'Al Raglo."

Rif and Rosh (ibid. and 12:7): The Yerushalmi says that the Ikar (primary part) of Chalitzah is untying the straps. The Halachah follows Ameimar.

Rambam (Hilchos Yibum 4:16): If he untied the shoe and she removed it, or vice-versa, the Chalitzah is Pasul.

Or Some'ach: We do not let her marry (a stranger) until she removes the shoe from most of the foot. However, perhaps untying the straps suffices to forbid her to do Yibum mid'Oraisa. We find that R. Eliezer requires spitting and removing the shoe to permit, but doing one of these forbids Yibum. A Tosefta (12:9) says that Chalitzah is Pasul if he untied and she removed, or if most of the leg came out, i.e. she untied and he removed. (Rivan in Or Zaru'a explains like this.) The Tosefta discusses Torah law. On 104b we say that when two things permit, one works (partially) by itself. However, this does not apply to one matter (Chalitzah, i.e. removing his shoe) which consists of two parts (untying and removing). Half a matter does nothing. Even though we find that if the Kometz caught fire, it may be burned at night, even though it does not permit the Shirayim until the majority catches fire (Menachos 26b), that is different. Once it caught fire, it is the Mizbe'ach's food. To permit the Shirayim, the majority must catch fire. The Gemara compares Chalitzah to Shechitah of the lambs, which permits another matter (bread). It must be done Lishmah; then, it takes effect on the bread. Also Chalitzah permits another matter and it must be Lishmah. Therefore, untying or removing by itself should not forbid. The Gemara (105a) concludes in the second version that the order of Chalitzah is not Me'akev. Spitting before removing the shoe is valid. If so, we can say that spitting by itself is a Matir (something that permits). At the end of Chalitzah it does not permit, because Chalitzah is strong and permits by itself. Spitting at the beginning is strong enough to forbid to the brothers. This is unlike Eimurim. The Mitzvah to burn them is Me'akev, but burning them before Zerikah is not a Mitzvah. Even though Ula says that if they came up before Zerikah Lo Yerdu, for they became 'food' of the Mizbe'ach, just like anything proper for the Mizbe'ach. The Mizbe'ach is Mekadesh it if no action need be done to it. Even so, we say that blood permits Eimurim to the Mizbe'ach (Zevachim 44a).

Or Some'ach: The Yerushalmi says that if he removed the shoe and she untied or she removed the shoe and he untied, he cannot return (to do Yibum), for they are not both in it (she did not do both actions), or they are out of order. These reasons argue about a case when she (previously removed and) later untied. Surely we do not discuss when he untied, she removed his shoe, and he put on a shoe and she untied it. Surely two actions on two shoes do not join! Rather, after she removed the shoe, the straps were still tied to his leg, and she untied them. This is not the way of Chalitzah. Or Zaru'a says that if she untied and he removed, she may do Yibum. This is unlike the Yerushalmi and Tosefta.

Rosh (ibid.): He must press his foot down so she will toil to remove the shoe. Sometimes Beis Din's shoe is bigger than his foot, and it seems that it comes off by itself. The Yerushalmi connotes that our Mishnah discusses when it is tied above or below the knee. Rabbeinu Nisim and the Rif say so. This is astounding. Why should it make a difference where it is tied? As long as she removes it (this is Chalitzah)! Their sandals could not stay in the foot without being tied. It says that he ties it until he can walk in it. This shows that tying is primarily so it will stay on his foot. Since it cannot stay on without being tied, if he untied it, it is as if the shoe fell off, for it is destined to fall off by itself. Likewise, if it was tied above the knee, the Ikar of what keeps it on the foot (the knot) is not "me'Al Raglo" (rather, it is on the thigh, which is me'Al me'Al Raglo). If a shoe stays on without tying, it is Kosher for Chalitzah even if it is tied above the knee or not at all. The Torah does not hint to tying, and there is no tradition from Moshe from Sinai about this. The Yerushalmi does not exclude one whose foot was cut off. Alternatively, it teaches that enough must remain below the knee to tie a shoe below the knee.

Tur (EH 169): If he untied the knot and she removed the shoe from his foot, or vice-versa, the Chalitzah is Pasul. The Rosh disqualifies when he unties only if he could walk in it only if it is tied. Then, untying it is like removing it, since it will cause it to fall off. If he could walk in it without tying it, and she removed it, the Chalitzah is Kosher, even if he untied it. If a shoe can stay on his foot when he walks without being tied, Chalitzah in it is Kosher b'Di'eved without being tied. The primary part of Chalitzah is untying and removing it. The Rosh says that he must press his foot down so she will toil to remove the shoe. Sometimes Beis Din's shoe is bigger than his foot, and it seems that it comes off by itself. The Rif and Rambam say that Chalitzah is Kosher only below the knee, i.e. untying. If it was tied above the knee, the Chalitzah is Pasul. If so, one whose foot was cut off, even below the knee, cannot do Chalitzah. R. Chananel says that the Gemara means that if his foot was cut below the knee, he can do Chalitzah.

Shulchan Aruch (EH 169:32): If he untied the knot and she removed the shoe from his foot, or vice-versa, the Chalitzah is Pasul.

Rema: Some say that this is only if he could not walk in it without tying it. If he could walk in it without tying it, and she removed it, the Chalitzah is Kosher, even if he untied it.

Beis Shmuel (31): The Rosh holds that if he can walk in it, it need not be tied, or it can be tied above the knee. We do not find any Posek who disagrees. We can say that they agree.

Prishah (56): Since it is Kosher if it was not tied at all, all the more so it is Kosher if it was tied and he untied it.

Noda bi'Yehudah (1 EH 94): It is normal to raise one's foot before removing the shoe. Only a fool would press it on the ground and make it hard to remove it! If so, lifting the foot is part of Chalitzah, just like the Rif says that untying the straps is a part. One lifts the foot after untying the straps. It is easier to untie while the foot is pressed on the ground. Therefore, if the Yavam lifted his foot, he began the Chalitzah, so it is Pasul. She must do the entire Chalitzah herself. This is why the Mechaber disqualifies when he untied the straps. The previous Halachah (Sa'if 32) disqualifies when he did not press his foot down. Both of these are due to the same reason. Since he did not press down, he began the Chalitzah. This answers the Beis Shmuel's question (why the Mechaber rules unlike the Rosh). I answer that the Mechaber holds like the Tur, who says that the Rif and Rambam disqualify (when he did not press down), and all the more so when he untied. The Rosh is Machshir when he did not press down, and also when he untied.

Hagahas Rav Baruch Frankel: The Rosh says that there is no Torah source or tradition that the shoe must be tied if it stays on without tying. In such a case, she needs only to remove it, therefore, he may untie it. This is unrelated to pressing the foot down. The Rosh holds that he must press because sometimes Beis Din's shoe is too big, and if he would not press down, it looks like it comes off by itself. The Rif and Rambam hold that he must press in order that his heel will cling to the shoe, in order to fulfill "me'Al Raglo." Seder Chalitzah (after Siman 169, Sa'if 66) says so. This explains our custom, that after he presses down, she holds his shoe with her left hand while lifting it with her right hand, so the shoe will cling to his heel even while she lifts it. Her right hand does the entire Chalitzah. Her left hand does not help to remove it at all; it only keeps the shoe clinging to his foot.