Starting Position: Stand with your feet hip-width apart holding a medicine ball with both hands directly in front of your chest; palms facing each other, and elbows fully bent pointing 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. Try to maintain a flat back by bending forward at the hips. Keep your head directly facing forward. Shoulder blades are pulled down and back and the abdominal / core muscles are engaged to brace your torso and support your spine. 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 up and over your head, bending at the elbows to take the ball back behind your head and neck. AT THE SAME TIME straighten your knees and hips, rolling up on your toes. The torso is leaning back and the hips are pushing forward to lengthen the abdominal muscles. Keep the abdominal muscles bracing throughout the movement to ensure spinal stability.
Explosively bring the ball back over your head to throw it directly down into the floor while dropping your weight back into your hips. As you throw the ball down, make sure you are keeping the abdominals braced, hinging 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.
If using a bouncing medicine ball make sure that you have ample space for safety; if you are using a non-bouncing medicine ball (a "dead" ball) make sure you are on a solid, stable floor that can support the force of the weight being thrown explosively into the floor.