Cable Crunch Correctly?

[wallcrawler]

Help me out! I need to know the proper way to do a kneeling cable crunch. I've been told to rest my butt on my heels and NOT to rest my butt on my heels. I've been told to keep my body upright and only curl down a few inches and only above the shoulder blades AND I've been told to curl ALL the way to the floor.

What is the correct form for doing this exercise?!!

 

[VNV_Disciple]

Here you go.

Cable crunch.