TT #636 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:03
Forty times he shall strike him; he shall not add, lest he continue to strike him beyond these with a great blow, and your brother be degraded before your eyes.
Torah Temimah points to Makkot 22b.
TT lemma: וְנִקְלָה אָחִיךָ.
English: ‘And your brother be degraded’ – if they assessed him as able to receive twenty, they strike him only with a number that can be divided into thirds (eighteen), but not twenty-one. What is the reason? Because Scripture says ‘and your brother be degraded’ – after he has been flogged we still require him to be your brother, and that would not be the case [if the number exceeded his assessed capacity].
Derash requiring survival after the penalty (Bavli).
The court never imposes a number that the assessment indicates would be fatal. The final count is always reduced to a multiple that can be proportioned (one-third front, two-thirds back) while preserving life.