btw-- when I said it looks like your chest starts to cave a little, I mean that when you sit up your torso doesn't go all the way up--- you reach forward with your neck/head to try to artificially create a tighter straddle, which bends your back (and thus caves in chest). This is doing yourself a disservice; you might get a couple inches further but it puts your torso/hips in a bad position to 'hinge'. This also keeps you making the sit-up part of it smooth; you can see in that video that you just aren't getting up high. You're clearly strong enough to do this but you're allowing your head to lead you. If you're walking and looking down, your back will arch and your chest will cave. Same deal here. Lift them to meet each other, don't just go forward and hope to get your legs high enough.
Yeah. So... not just the hip/leg part, both halves need to work together to close the angle.
(All Jon's advice completely from Jon's POV and experience and not affiliated with GBodies certified instructors/athletes
no promises this is 100% correct or what you need to focus on)