Why do we exhale before doing reps in ab exercises?

Question

Normally, we inhale before doing a rep in the typical lift. However, when comes to abs eg: wood chopper, crunch etc, it is often advised to exhale before doing the rep. Why?

Answer 1

Inhaling increases intra-abdominal pressure, increasing the rigidity of the trunk.

If the point of the exercise you are doing is to have a mobile spine, then increasing the rigidity of the trunk can work against that.