Hello again BL
The exhales in Sit-Up serve good purpose. There is of course the cleansing effect of the forced breath. Your first exhale is functionally the most important. It is the one that helps you bring yourself off the floor to vertical: your “L-sit” position. It helps bolster support to your lower spine to get you up off the floor safely.
Because your abdomen is largely fluid filled you need to brace everything otherwise it is as if you have a whole lot of fluid sloshing around in there. When you ‘suck in your stomach’ there is hydraulic pressure of the organs against the lower back muscles etc (and you get some control over the effects in this area). The organs ‘sit’ firmly against these structures and lend ‘strength’. This a part of what we mean when we talk about your core muscles. Core muscles are more than that as they include more than just those muscles that ‘suck in your stomach’.
It takes training to involve those deeper core muscles - to activate the muscles around your perineum. Many people do that automatically or develop them to some extent through their Bikram Yoga practice. Perhaps the subject of another topic!
The second exhale happens from “L-sit” to the forward sitting position as you extend your lower spine and sit forward (with a straight back!).
As far as speed is concerned. You will probably find that when you break the movement up into 2 distinct parts your Sit-Up will be a little slower and more deliberate (safer, more beneficial and more enjoyable too). Those who fling themselves up bypass “L-sit” and use different muscles. Their Sit-Up is quicker, they have less support for their lower spine (less safe) and the double exhale occurs at non-specific points. Enjoy slowing it down a bit, reach up to the ceiling as you finish your first exhale.
Kind regards
Gabrielle