Left hand radical - side-form of
手 (Kangxi #64). Anchors
挞 in the hand-action family.
挞 specifically marks a striking motion - to whip, flog, lash. Sister striking-verbs:
打 (hit),
拍 (clap),
揍 (beat up),
抽 (whip with a switch). The radical states 'something done with the hand' immediately.