The way I've heard it described is you let your belly expand all the way out with a big breath of air, then clamp your abs down tight, so you're squeezing on that air pressure and stabilizing the spine.
I agree with Thomas. When wearing a belt, some will teach to push out against it, but I find that that happens naturally when you just brace the same way bc the belt is tightened a bit inside the normal brace circumference.
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