This is really unbelievable nonsense, because you hear this very often. You don't breathe with your stomach, so putting weight on your stomach makes no sense. Because you don't breathe with your stomach, you breathe with your ribs and your diaphragm. And because of that, the ribs move to the side and upwards, so you need to learn to make your ribcage flexible. Nothing to do with abdominal breathing. Of course, if you push your belly forward, then the diaphragm moves down somewhat, because the space has to go somewhere. But the diaphragm is not actively used. This way of breathing is totally unsuitable for overcoming stuttering. It's a disgrace that this is still taught. This is the great misunderstanding, and you're perpetuating it.