Underhand grip
Short definition
Supinated (palms-up) grip: the palms face toward the body / upward.
Full definition
The underhand (supinated) grip is the mirror of the overhand grip: palms face the body. It does not change force transmission through the bar (physics), but shifts the muscle load: in a bench press or bar hold it moves the load toward the upper chest and anterior deltoids and recruits the biceps as a stabilizer; in a bent-over row it increases biceps and lower-lats engagement.
Examples
Bent-over row with a supinated grip — more biceps involvement.