Musar for Keritot 49:10

שחיטה שאינה ראויה שמה שחיטה

Now, according to me who hold it is forbidden 'in its lifetime', [the meaning] is clear, for the dispute between R'Simeon and the Sages lies in thi R'Simeon holds that ineffective slaughtering<span class="x" onmousemove="('comment',' I.e., slaughtering which does not render the flesh fit for use. According to R. Hamnuna, the heifer whose neck is to be broken is forbidden when still alive, and its slaughtering is indeed of no avail with regard to rendering the flesh fit for use. R. Simeon holds that such slaughtering does not come within the scope of the prohibition of Lev. XXII, 28, whilst the Sages hold that it does.');">⁵</span>

Mesilat Yesharim