Starting Position: Stand with your feet hip-width apart. Hold a medicine ball in both hands directly in front of your chest; with your palms facing each other, and elbows pointing down toward the floor.
Downward Phase: Shift your hips back and down. This will create a hinge-like movement at your knees. Continue to lower yourself until you feel your heels about to lift off the floor. Pull your shoulder blades down and back. Engage your abdominal / core muscles to brace your torso and support your spine. Bend forward at the hips, keeping the back flat and your head facing forward. Reach the medicine ball back through your legs, preparing to bring it up overhead rapidly.
Upward phase: From the lowered position, push your feet into the ground and explosively swing the medicine ball up and over your head all the way to the point where it is behind your head and neck. Throw the ball back down to the ground behind you. Your ankles, knees and hips will all completely extend. You will be reaching back and pushing the hips forward to lengthen the abdominal muscles. Keep the abdominals contracted and braced throughout the movement to ensure spinal stability.
When doing this exercise indoors be sure that you have enough space behind you to throw the ball into the ground (similar to doing a reverse slam dunk in basketball); it is recommended to use a 'dead' ball that does not bounce.