Knees and chest meet in the middle.
Squeeze upper and lower abs together at the top, don't just rock.
The tuck crunch attacks the core from both ends at once, your torso curls up as your knees pull in, so the abs shorten through their whole length instead of just the top half. Keeping your feet off the floor between reps means the tension never switches off. It earns its place as a step up from the standard crunch that still needs nothing but floor space.
Two things, the double movement of ribs and knees meeting in the middle, and the fact your feet never touch down. That constant tension is what makes short sets burn.
You are pulling on your head instead of curling with your abs. Keep your hands as a light shelf behind your head or cross them over your chest, and lead the crunch from your ribs.
They handle the trunk flexion box well, but a complete core routine also wants anti-rotation and stability work like planks or bird dogs. Pair them and you have most bases covered.
