FAQ for
Half Nelson (2006) More at IMDbPro »

It's a term from wrestling- when a wrestler, from behind his opponent, passes one arm under the corresponding arm of his opponent and locks the hand on the back of his opponent's neck.

The particular move is similar to the act of strangling, which represents the main character's situation. Also, it's a fairly difficult move to get out of without putting up a real fight, which again resembles the drug addiction.