An important caveat is that this is primarily a hip flexor movement, the legs moving relative to the hips. It's only in the top portion of the movement when the hips naturally move relative to your upper body that the upper abs are being used. So a common cue is to lift your butt up instead of your legs
Yes. This exactly. Hence my focus on keeping my legs as straight as I can each movement. Keeping them completely locked out decreases blood flow but pushing up with glute muscles helps keep the balance through the motion. I should have mentioned I find keeping my elbows bent near 90 helps that way I'm weighted more on abs than shoulders.