You perform the exercise by lying on your back with the shoulders/upper back and neck supported by the ball.
The hips should be up and the glute muscles activated so the upper body is in a nice straight line from shoulders to the knees.
From this starting point, raise the weights above you until your arms are straight. Maintain straight arms as you slowly drop the hips straight down toward the ground. Once at the bottom, lower the weights, then press them overhead as you raise your upper body back to the starting point in the straight line.
While the move may seem simple, there is a lot going on that gives you a great workout in a short period of time.
There are a few key points to remember:
1. Keep your tongue on the roof of your mouth. This turns on the neck stabilizer muscles and will help protect your neck. You don't have to press hard with your tongue, just swallow and let it go into place naturally.
2. The core work comes from holding up the weights while dropping down the body. Make sure you keep the arms straight don't let them bend and drop the upper body down slowly. Going fast is cheating.
The core work is what makes this exercise such a high value because the core is working "under load." It is much more functional than doing isolated core exercises because it is a movement that translates into life or sports, where you are pushing, pulling or maintaining weights.
3. The lower back and legs also will get plenty of work from the dip. The key on this part is getting the back to hinge and fold down toward the floor as close to the ball as possible. If the ball rolls forward, that means your quads and hip flexors are doing most of the work, not the low back, so try to keep the ball from rolling as much as possible.
4. Finally, the press works the chest. Doing a press on a ball makes the body balance more than a bench, so don't be surprised if you can press less weight than you thought, especially if you aren't accustomed to using a ball in weight lifting.
Enjoy this simple exercise that gives you a lot of bang for your buck.