You have to listen hard for a great breathing tip for ab-mat sit-ups. There is a closer, louder breath video on my Instagram account @pelvicwod (its a black and white video). Breathe in to reach back, start breath out just before lifting off ground and all way through to touch ground in front. If you can coordinate it, connect into deep abdominal and pelvic floor muscles with your exhalation.
This may not be appropriate if you have pelvic floor muscle dysfunction - make sure you are seeing a qualified physiotherapist in this area if you are unsure or need help.