Ohr Chayim on Holy Sparks says that there is a type of holy spar that is attached to the Qelipah and which cannot emerge at all. It cannot develop into a holy soul that is born and eventually joins the Jewish people. Rather it is enmeshed within the Qelpah and it has no way to separate from it other than through the Jews being pained through the suffering of exile OR through the blows inflicted upon them. Through this suffering the holiness within these nations can be extracted and returned to its root to become part of the Jewish people. See [[Az mi-lifnei vereishit]].
Ramchal says in Ma'amar HaGeulah Siman 36 says that all the sparks of holiness must be returned to their original place, then the exiles will be gathered in.
The greater sparks of Holiness have been completely removed from the Qelipah and the only ones left are those in the heel. Sar haPanim is the Yod taken from Esav's heel which redeems the sparks from Qelipah as when the "feet touch the feet".