Wear them as Tefillin between your eyes (6:8)
The Mishna in Pirkei Avos teaches (2:4): 'Do His will as your will (Turn Hashem's will into you will), so that He will make your will like His will. Nullify your will in the face of Hashem's will so that He will nullify the will of others in the face of your will.'
This Mishna can be understood with the following Medrash (Vayikra Rabba 21:5). The Medrash says, if a person did many sins, to counter them, he should do many mitzvos. The Medrash gives the example of one who sinned with his eyes, he should improve his observance of the mitzva of Tefillin (which the Torah writes is to be worn "between the eyes").
This Mishna can be understood to be referring to someone who sinned with numerous sins and the Mishna is directing him on how to repent for his many sins. The Mishna says, 'Do His will as your will', this means one should serve Hashem (do His will), the same way he previously served His Yetzer Hara. When he listened to his Yetzer Hara he did multiple sins. Now, when he wants to repent, he should do the will of Hashem in the same manner he did his own will (listening to the Yetzer) – by doing many Mitzvos.
If one does so, the Mishna continues– 'so that He will make your will like His will'. This means that if one does Teshuva in the aforementioned fashion, then Hashem will turn the sins into merits. This concept, the Gemara (Yoma 85b) teaches that when one repents out of love of Hashem, his sins are turned into merits. The Mishna now reads as follows: (repent by doing many Mitzvos in place of the many sins that were committed) 'so that He will make your will' – your original will, your sins that you committed before repenting, 'like His will', Hashem will turn those sins into Mitzvos (His will).
This only happens if one repents out of love Hashem. Whereas, if one repents out of fear of punishment, then his sins are looked at as inadvertent transgressions, but they are not raised to the level of Mitzvos. Therefore, the Mishna ends off with – 'Nullify your will in the face of Hashem's will so that He will nullify the will of others in the face of your will'. This is to reiterate that the Teshuva must be done out of love.
The Mishna means as follows. One must nullify his (original) will. This refers to his original sins that are referred to as his will. To nullify his will (his original sins) would only be possible through repenting out of love since if he were to repent out of fear those sins would not be totally nullified. They would remain inadvertent sins. If he does so and repents out of love, the Mishna continues – 'so that He will nullify the will of others in the face of your will'.
This means that in the merit of the Teshuva out of love, the will of others – the prosecuting angels – will be nullified. They will no longer be able to testify against this person. The reason is – 'in the face of your will'. Since the person has now changed his will from a sinful one to one that does many Mitzvos (both because his repentance turned his sins into Mitzvos and because he now does many Mitzvos instead of many sins) they will not be able to prosecute him.
