The way Rav Shach sees things is that there are two different kinds of "pushing off" in terms of sacrifices. That explains the law of the sages (against R. Elazar) that when one sets aside a female pregnant sheep for a passover, the baby sheep and mother must be put to pasture. [That is so even though in terms of a sin offering עובר לאו ירך אמו a fetus is not part of the mother. That is: if you sanctify a pregnant female sheep or cow or goat as a sin offering, she or her infant can be brought as a sin offering--but not both.] So this looks like a direct contradiction..
The answer of Rav Shach is that the Rambam goes like the opinion בעלי חיים אינם נדחים but in the case of the female sheep, the essence of the holiness is already pushed off. It is a different sort of being pushed off.
That is where he disagrees with Rav Isaac Zev Soloveitchik. R.I.Z. wrote in order to answer for the Rambam that even though בעלי חיים אינם נדחים still when one sanctifies a female sheep that is pregnant, the fetus is one sacrifice with the mother. Rav Shach asks on this.
The regular case of pushing off is when a sheep is owned by two owners. and one sanctifies his half, then buys the other half and sanctifies that. The sheep can be brought as a sacrifice. That is obviously different from sanctifying a female sheep for a passover--which must be a male sheep.
If you consider Rav Isaac Zev' answer, it does make some sense. But the answer of Rav Shach makes much more sense since it takes into account something apparently unnoticed by Rav Isaac Zev, these two kinds of "being pushed off".And besides that Rav Isaac Zev's way seems like just away of sneaking עובר ירך אמו into what is in fact the law that עובר לאו ירך אמו
