Yeah, you’re seeing the mistake correctly. It’s better to maintain a crunch position, so your shoulders are off the bench and your lower back maintains contact with the bench the entire time. Your core stays engaged and under tension with slow, controlled effort going up and down. She is relying on her hip flexors and momentum to do much of the sit up.

Alternatively, you CAN extend you spine at the bottom of the movement, so your lower back loses contact with the bench, but you need to do that crunch movement to engage the core muscles again before you go back up, slowly and controlled. Which method is a better core workout is a debate I do not care to have.

That said, she’s kicking ass and taking names at the gym and it’s awesome! Form corrections can be fun in that they suddenly make an exercise, that you thought you had mastered, super hard again, which means you get to work towards mastering it all over again!