So here's what happened. I was practicing my daily Farinelli breathing exercises (basically breathe in for X count, suspend your breathe for X count and exhale for X count, increase X count by 1 and repeat) and I noticed that I can actually exhale and inhale, simply by consciously expanding/contracting my belly. And when I do that, I don't feel any air come in or out of my mouth.

Usually when breathing, I think of it in terms of taking inhaling/exhaling from my mouth, but is that necessarily correct? Should I think in terms of expanding/contracting my belly?

My guess is that it's actually a co-ordination between the two, however I wonder percentage-wise which would be doing the most work. I'm guessing that it's a "balance" between the two.