Shalom Avraham,

The Chachamim and Rebbi Shimon disagree about how to interpret the word "mi'Heyos."

The Chachamim understand the word as if the letter "Heh" functions like a "Ches," reading it as "mi'Chiyos," from the root of Chayim, life. In other words, as long as the lamb is alive, people may withdraw. Once it is no longer alive, meaning after the Shechitah, withdrawal is no longer possible.

Rebbi Shimon understands "mi'Heyos" from the root of "Havayos," meaning the events or developments that take place with the Seh. The root "Hayah" is often used to describe the unfolding facts or narrative of something. In the case of a Korban, the "Havayos ha'Seh" would refer to the Avodos performed with it as a Korban. According to this understanding, as long as the Avodos have not been completed, withdrawal remains possible.

The Gemara clarifies that the disagreement is not about joining the Korban, but about withdrawing from an existing Minuy. According to Rebbi Shimon, one may withdraw until the Avodos are finished.

This brings us to your second question. Conceptually, according to Rebbi Shimon, withdrawal should be possible until the end of the Zerikah, since the Avodah concludes with the Zerikah. However, the Korban Pesach requires only a single Zerikah on the Mizbeach, as written in Perek Eizehu Mekoman (Zevachim, chapter 5). The time between the beginning of the Zerikah and its completion is extremely brief. Practically speaking, there is almost no measurable difference between saying "until the Zerikah" and "until after the Zerikah."

On a conceptual level, it would seem that "Ad she'Yizrok ha'Dam" means until the Zerikah is completed, just as according to the Chachamim, "Ad she'Yishachet" means until the animal is slaughtered and no longer alive.

Kol Tuv,

Aharon Steiner