Most BB'ers will lack the shoulder/joint flexability when trying behind the neck/head movements for the first time. Tightness limits the ROM to do the exercise correctly. Might want to apply stretches and good warm-ups before lifting. Hanging from a chinning bar, from a wide to narrow grip, can reduce tightness. Do not do any pull-up, just hang and relax the shoulders for a minute or so. Straight arm pullovers, with a close to medium grip are also good. Though too wide a grip on the pullover can cause shoulder joint strain. Pressing a light bar(exercise bar) behind the neck/head for many slow, controlled reps helps with flexability during a warm-up. Even try some light Bradford presses (front to back, etc) for reps.
Behind the neck chins (or pull downs) affect the back very well. Hit's my posterior delts, along with the lats....for me any way. Touch the nap of the neck each rep. Of course, add weight when needed. The press behind the neck is a superior exercise for the laterial head, and to a lesser degree the anterior head. Lot's of weigh can be built up with these. Do not go too far down the back but just touch the mid to upper traps. Going down too far can have a negative affect on the shoulder joints. Press behind the neck's seem to improve the benching. It gives strength to the whole shoulder girdle when done the correct way. Lot's of weight can be handled in this exercise, either sitting or standing. Good Luck.