Rashi citing a Midrash: If you slackened from Torah, the angels will not invigorate your strength.

Rashi: You slackened on the day of your beloved's affliction to stand opposite (you did not come to help him - therefore) your strength will be weak on the day of your punishment - 'Im Hacharesh Tacharishi..." (Esther 4:14).

R. Yonah #1: Verse 9 discussed the sin of Letzanus. Our verse teaches that there is a punishment for slackening from good thoughts where needed.

R. Yonah #2: This refers to "v'Ish Da'as Me'ametz Ko'ach" (5). He invigorates the weak at a time of affliction, via counsel and Da'as. If you slackened and did not put to your heart to save lives - to think of a counsel to save them from their affliction - your strength will be weakened and you will be punished for this. They will measure to you Midah k'Neged Midah. You have the ability to save and invigorate strength, with the Da'as with which Hashem graced you, and you slackened, and made yourself like a weakling. Therefore you will be weak in your deeds, and not be able to act valiantly. It should say Im Hisrapisa; the word Im is omitted, like "v'Azav Es Aviv" (Bereishis 44:22) - if he will abandon his father.

Malbim: If you will slacken from saving those taken to die, also when they are about to be killed (11), your punishment will be Midah k'Neged Midah. When a day of affliction comes upon You, you will slacken, and your strength will be weak. Your hands will not be able to save you, and you will have no strength to do anything, just like you did not save others.