What do we learn from "Yiten l'Makehu Lechi"?
Ibn Ezra: He should let him strike him.
Sha'arei Teshuvah (1:28): One who is Ma'avir Al Midosav (does not retaliate at those who pain him), all his sins are pardoned, Midah k'Neged Midah. This is a very dear opening of hope [to merit in judgment]!
Palgei Mayim: Because he believes that all is from Hashem, any affliction is not difficult for him.
How will he be satiated with shame?
Rav Sadya Gaon: [The one who struck him] will revile him greatly.
Erech Apayim 1(83, citing R. Avraham Balanti): Bearing insults is the greatest Teshuvah, more than all afflictions and fasts. He is satiated, does not get weak and he is not Batel from Divrei Torah.