That is good that you are already a belly breather. That is the first step. The next step is to learn how to brace and breathe at the same time.

Most of the time when people brace, they have to flex their entire muscle set in the abdomen including muscles they would breathe with. So they end up holding their breath in order to brace.  In actuality you only need to brace with (i think – my anatomy is not really up to par) the transverse abdominals and not with the rest of the muscles there.  This is how you brace 20% all day long and not feel like you can’t breathe.  Of course in times of stress, like when you’re under heavy load, you’ll need more of your abdominal muscles to hold your spine steady – then its more like breathing with a lot of shallow puffs.

Then when people start to breathe, they end up letting go all their abdominals and then their lumbar spine gets overextended as a result.

I have found a good chapter on abdominal breathing in Rehabilitation of the Spine by Craig Liebensen, but am still looking for texts on the other elements like breathing pattern and rate and breathing while bracing. I’m about to take a class that has some good rehab techniques in doing breathing while bracing.