Holding your breath during the upward movement of curl-ups is acceptable if a full exhale cannot be achieved, and releasing air can enhance abdominal contraction intensity.

“if you find that you can't do the big exhale as you bring it up to the top don't worry about it it's perfectly fine to hold your breath as you come up and then just maybe release a little bit of air to enhance the uh intensity of that abdominal contraction”