my understanding is that the vac pose is muscle control and breathing. You kinda want to pull your organs up under the rib cage. Arnold describes practicing the exercise in the encyclopedia of bodybuilding as very difficult to master, but you want to try on your hands and knees...sucking in your gut and trying to pull your organs up with your diaphragm into your ribs. Then you breathe out and try to pull them up even higher. You also need to have low bodyfat and good tone in your abs to master this. Not like it will ever be called out in prejudging, but you could wow some judges if you popped it while setting up a hands over head abdominal/thigh pose.
guys like zane and arnold were great at this, but i doubt you will see anyone these days try it on stage with their GH bellies.
