Speed's the name of the game in fast pitch softball. Top pitchers hurl the ball across the plate at more the 60 mph, always delivering the pitch underhand. The windmill style of bringing the arm around one full revolution before releasing the ball is favored by many. Some leagues allow different styles, but the Amateur Softball Association has general rules that apply to all windmill pitching.
The Windmill
The windmill motion must be less than two full rotations of the arm. The windmill motion must stop after the ball is released.