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[19a bottom] R’ Yehudah: If one immerses in a minimally Kosher Mikvah of 40 se’ah and comes out wet (thereby decreasing the necessary required amount to an un-Kosher Mikvah), but is still standing with his feet in the water, and someone else subsequently immerses in the Mikvah, they are both considered purified (because the exiting of the first is not yet complete).
Kesef Mishnah: Purification from a Mikvah only takes place once one exits the water completely (not while still immersed).
Sefer Hachinuch: The Mikvah works as a re-birthing process. The Holy waters that we enter, bring us back into the fluids in the womb inside our mother and when we exit we come out clean and pure again as new.
source site E’s Lesson: This is one of the three ideas we should keep in mind during Mikvah dipping. Hashem, through His graceful mercy, is allowing us to hit “restart” and live a pure fresh new existence. Through proper Teshuva, we can reconnect to our Parent and step out reborn into a world with brilliant positive opportunities.