The purpose of a kneeling ab cable crunch is to target the rectus abdominis with spinal flexion. It should be done with a "rolling of the spine" rather than just hip and trunk flexion.
Compare this to the most common (and incorrect) way they're done: with a bending of the hips while sitting the butt back toward the heels.
Can't stop doing it that way? Try this instant fix for better cable crunches:
What to Do
Crush a large medicine ball between your butt and heels. That's it, literally.
The simple act of barricading your hips from being able to sit back will instantly put you in the correct position. Even if you wanted to, you couldn't get it wrong.
Just be sure to use the right size ball (a trial and error approach is best) then wedge it in there good, squeezing it as hard as you can by activating your hamstrings.
Why It Works
The ball cues correct body positioning and stops unwanted movement of your hips. Limiting hip and knee flexion creates a stable platform from which you can better isolate spinal flexion. You can sort of compare it to what a preacher curl might do for your biceps.
By activating your hamstrings you'll also get some reciprocal inhibition of your hip flexors. This means you can better isolate your superficial abs and make them work harder.
Doing cable crunches like this for the first time is an epiphany moment for many. You'll never crunch the same way again.