Fold into a V and your abs have nowhere to hide.
Raise both halves together and balance on your glutes at the top. No dropping your legs.
The V up demands that your abs move both ends of your body at once. Lifting torso and legs simultaneously to meet in the middle works the entire front of your core through a long range, and lowering back down without touching the floor keeps the tension constant from first rep to last. It sits above crunches and leg raises in difficulty precisely because neither half of the movement gets a rest.
Build up with tuck ups, bending your knees as you fold so the lever is shorter. Once those are easy, straighten one leg at a time, then progress to the full V with both legs long.
Your lower abs are giving out first, which is normal. Slow the lowering phase right down and focus on both halves moving together, even if that means fewer reps.
They are harder, because your legs are part of the load instead of anchored to the floor. If sit-ups have stopped challenging you, V ups are the natural next step without adding any equipment.
