The driving hip (the leg that is contact with the rubber) must be square on release on the ball. The rule is the same for 9-Man and 10-Man.
The easiest way to tell if the driving hip is square is to look at the pivot foot. If the pivot foot is pointing towards home plate, then the driving hip will be as square as possible. If the pivot foot is angled towards third for a right handed pitcher or first for a left handed pitcher, the driving hip will not be square on release.
