My two penneth is regarding loop volume.
On a standard inspiration RB (without an auto-diluent valve), as you descend, the loop volume will decrease as the ambient pressure increases, meaning you can get less and less of a breath. You would normally press the diluent add button on the counterlung to add fresh gas to counteract this.
If you have an ADV fitted, or are using a KISS
, you just take a breath and it activates it, adding fresh diluent into the loop.