Starting Position: Stand with your feet hip-width apart and holding a medicine ball in both hands directly in front of your chest; grasp the ball with your hands in a neutral position (palms facing each other), and elbows fully flexed (bent) and facing down towards the floor.
Downward Phase: Begin your downward phase by first shifting your hips backwards then slowly moving downwards to create a hinge-like movement at your knees. Continue to lower yourself until your feel your heels about to lift off the floor. Depress and retract your scapulae (pull your shoulders down and back) and engage your abdominal / core muscles ("bracing") to stiffen your torso and support your spine. Maintain a flat back by bending forward at the hips, keep your head facing forward and reach the medicine ball back between your legs to prepare to bring it up overhead in a rapid motion.
Upward phase: Swing the medicine ball upwards over your head while reaching it back behind your head and neck and simultaneously straightening your legs and rolling up on the balls of your toes to create triple extension (ankle plantarflexion, knee and hip extension); lean back while pushing the hips forward to lengthen the abdominal muscles, keep the abdominals contracted and engaged (bracing) throughout the movement to ensure spinal stability.
Explosively bring the ball down over your head to throw it directly downwards into the floor while dropping your weight back into your hips. As you throw the ball down, make sure you are maintaining the abdominal bracing and hinge from the hips to maintain spinal stability.
Exercise Variation: Use a bouncing medicine ball to throw it straight down and have the ball bounce back up into your hands to perform continuous repetitions.