Once More, With Feeling - S6-E7
Corrected entry: During Buffy's training montage, she is seen punching a bag with her wrists bent. First, that's going to be a weak punch, super strength or no, because she can't transfer the force from her arm and body; it just bends at the joint. Second, that's a great way to injure your wrist, because all that escaping force snaps the bend farther. You could strain or even break the joint. After six years, Buffy should know better. (00:31:50)
Correction: That may well be true, but Buffy has superhuman strength. So, it wouldn't hurt her hands and would still pack a big enough punch.
THGhost