start from about 6 feet in front of the rubber. stand at a spot that is a 45' angle to the pitching rubber to the glove hand side of the mound. step toward the rubber with your glove hand foot and jump off that foot planting your throwing hand foot (the normal post foot) against the rubber in your normal pitching position. allow the back leg (post leg) knee to flex and catch your weight when you jump into the rubber.
when you feel the weight and momentum stop into your back foot, push off the rubber jumping horizontally toward the (plate, catcher, target), get the stride foot down and throw the ball. you should feel an enormous amount of momentum and energy going to the plate to throw the ball. learn to generate this type of force and momentum in your wind up and control it until release, and you will throw as hard as your body is built to throw.
hope this helps.