Starting Position: Make sure that you are working with a partner who can catch the ball, let the partner feel the medicine ball first so they know how much it weighs and can prepare to catch it. Position your body directly facing your partner with the spine in neutral (normal-to-flat), shoulders depressed (no shrugging) with scapulae retracted (squeezing shoulder blades together). Keep your chest held up and out, head in line with your spine or slightly extended (raised), heels on the floor. Stand with the feet hip-width apart, the left foot slightly forward, the right foot slightly back while holding a medicine ball between both hands; contract and brace the abdominals while depressing and retracting your shoulder blades. Your eyes should be looking directly at the target in order to properly aim the throw.
Keep your feet pressed into the ground, continue to contract and brace your abdominal muscles, swing your arms upwards to lift the medicine ball overhead and bend your elbows to reach the medicine ball behind your head. As you lift the ball over your head, shift your weight back into your right leg and lean back to engage the core muscles for extra power for the throw. While preparing to throw the ball aim the throw by keeping your eyes on your partners hands as you pull the ball back over your head.
Explosively step forward with the right leg and quickly bring your arms forward over your head to generate power for throwing the ball. While stepping forward with the right leg, push the left leg into the ground create triple extension (ankle plantar flexion with knee and hip extension) and generate more forward momentum for the ball.
Exercise Variation: Repeating throws: Use a wall with a medicine ball capable of bouncing and position yourself so that you can throw the ball into the wall and quickly catch it to prepare for the next throw.
The purpose of this exercise is to generate upper body power, use a ball that is no more than 10% of your bodyweight to ensure that you can acheive the appropriate velocity for the throw.