The pike pushup is an indispensable drill for working your way up to freestanding handstand pushups. Not only does it help you build enough shoulder strength for HSPU’s, but it also improves your balance during the descend.
The major points here are to learn how to pull your shoulders towards the ears in the start position and activate your lats on the way down while pulling elbows towards each other.
Check out Hand Balancing For The Bodyweight Athlete by Global Bodyweight Training if you’d like step-by-step instructions for mastering various hand balancing styles.