Neck cranks are dangerous man, I have been hurt by them. I never use them on people, never really have to, cause I practice with guys that are all in their 30's and we have to be to work the next day so we don't try to kill one another. We call this happy-jiu-jitsu. Tourney jj pull his fucking head off if you can. Best used when in someones guard and you can't get out. Reach up with both hands, interlock your fingers at the crown of his head ( they will teach you to grab behind the head but pull from the crown or top) and pull his chin down to his fucking bellybutton and hold. He will more often than not break his guard or give you his credit cards pretty much anything you want.
I do over head close cable grip pulldowns to train for this. I can pull the stack with a 45 lb. plate hanging on the pin for 8 on my last set. If I had too I am pretty sure I can snap someone's neck. But be judicious with these you don't want to kill people.