This drill is about you becoming better at recovering an undershot kick up.
Prime yourself in doing things wrong by performing an undershot rep first, then kick up again: the moment you feel the undershot, bend and extend the elbows - performing in essence an elbow elevator - trying to recover from this mistake.
You can practice this back on the wall first and eventually freestanding.

⚠️ careful we ideally want the elbow flexion to be reactive not proactive this is not the same as you kicking already into a bent elbow position.