I couldn't find a "rule" online, however, there was plenty of talk from coaches etc... Seems it is stuff like face locking or cranking necks either choking or steering them by the neck.
"Any unnecessary contact with the head or neck in effecting a tackle, particularly where there is a second movement to make contact with the head or neck involving a jerking, twisting or pinning action."
That is the definition but as OB said it is basically using the forearm or bicep across the neck like a choke. I have noticed over the last few seasons players beginning to use wrestling/grappling techniques to tackle and slow the play the ball. They try hard to put players on their back, turn them backwards and keep their hips low and apply pressure with the shoulder when the opponent is down.
Melbourne, Brisbane and the roosters all did this early so I would certainly say it has something to do with the tackling coaches.
I play league and coach some teams wrestling with my background being BJJ. The main concern of the grapple tackle is the neck crank type but any controlling of the head with your arms and heads is a penalty now. As they are reafing peoples heads and cranking there necks in a fairly uncontrolled environment . I can't believe it took them so long to outlaw it properly to be honest. I would hate to have a boofhead cranking my neck while another is pinning me and possibly another one trying to turn me onto my back all the while I am holding a ball and can't do anything because I am not supposed to let it go. Alot of the focus I put into the teams I am training is still standard control methods we use like underhooks, head position, closing space between yourself and the other person for both standing and on the ground.
Everything is done while you are moving at a fair pace and small adjustments have to be made because the opponent has a ball which makes things a bit harder for both tackler and runner. I find in the wrestle on the field I can dominate a guy running the ball fairly easy because of the wrestling instincts, by controlling there upper body with well placed underhooks you can put them anywhere you like. You can even have a bit of fun on the ground now and them as well. I find the funniest thing is when I go to hands and knees to play the ball and one player drops on top of me like in north south to gut wrench me, I sit out to the side classic BJJ style and because they are clenching so hard, they flip on there ass as I sit out, it leaves them wandering what just happened.