I think throwing from an overhead release (like the standard baseball pitch) is a humeral flexion thing. Humeral flexion is trained by chins, pull-ups, pullovers. However, it also has a triceps extension component to it.
On another note, I have been doing lots of weighted chin-ups lately, and a volleyball serve is the same anatomical motion as one. I decided to test myself and see how hard I could hit it and ended up smashing it against the wall. Someone asked me if I went to weight-room for volleyball or something....hahaha.