This is an excerpt from Ultimate Conditioning for Martial Arts.
This is a fantastic exercise for developing upper-body power in extension (as in a strike) and for complementing shoulder-integrity work.
Medicine ball (preferably sand filled)
Grab a medicine ball (preferably sand filled as it will provide more stability for the exercise) with both hands and hold it in an extended position while in a narrow push-up position (figure 5.22a). Descend rapidly and bring the chest to the ball (figure 5.22b). Reverse direction explosively and ascend as high as possible (figure 5.22c). For those with good upper-body power, it is not uncommon for the ball to leave the ground. However, the real beauty of the exercise lies in the stability that is needed once you and the ball return to the ground. At impact, maintain a braced core and stay stable through the shoulders. Keep repetitions in the range of three to six for two or three sets, depending on where you are in your training program.
Medicine-ball jam: (a) start position; (b) descending and bringing chest to ball; (c) ascending explosively as high as possible.