In a method, which I will call the Hold Method, it doesn’t matter if you are inhaling or exhaling, you simply hold your breath, tightening your stomach muscles. This allows you a certain amount of freedom in your ability to attack in a timely manner. That said the idea is not to harden or tighten your stomach muscles unnaturally, but if you imagine pushing or pulling something with about 80% of your strength you will naturally tighten your stomach muscles to help.
It is the natural tightening of the muscles that is used to increase strength over short durations. This is opposed to the idea of an exhaling breath, which is designed (it seems) to keep your body tight and increase strength over slightly longer durations typically. (Unfortunately, this is unnatural and must be learned over a great deal of time, with ever increasing levels of stress to be even slightly productive. Of course, it’s no enough that a person have greater and greater planned stress because it must also involve the element of fear and the adrenaline dump that comes with it to be truly reminiscent of combat.)
In addition, it is not the type of stomach contraction that you might produce as if trying to push or pull something with all your might, but rather to have a sort of “relaxed tension.” That is, that your core is strong but not absolutely tightened. By holding your breath this way during your attack, it can be used to for continuous attacks rather than one single one.
More to come...