Hmm... interesting... I've always been taught that the neckreining controls the front, the heels the back. Mind you, I learned everything I know at a dude ranch where finess means nothing and getting the job done is everything lol
It is where you PLACE your heel that determines what part of the horse moves.
So let's say you want to do a rollback where the horse plants their hind end and turns their front end. Yes, you can accomplish a rollback by neck reining only (and no other cues) but it is going to be much more efficient for the horse and lighter in your cues for you if you do all cues: weight, leg, rein, seat. So if we are turning to the left, you want to keep your left leg completely off the horse (to open the door) and you want to slide your right leg slightly forward to cue the front of the of the horse to turn.
If we want to do a turn on the forehand, you won't use the neck rein cue as much as a rollback, but it still reinforces what you want. But you are going to move your right leg back on the horse (if we want them to move their hindquarters to the left) to encourage the back end to move and again keep your left leg off them.
If you want to turn both the front and back simulanteously (so that we are doing neither a rollback nor a turn on the forehand) you would use one leg slightly forward and one leg slightly backward to correspond how you want the horse to turn (left or right).
But the main point I am trying to make is that you should NEVER be only giving your horse ONE cue only (such as a neck rein only). You would always be using all the cues you have at your disposal --> seat, legs, weight, reins.
Maybe an example of a car would help:
What do you do when you make a left turn in your car? You take your foot off the gas, apply the brake, turn on your left signal, and turn the steering wheel. You use many cues!
It wouldn't make sense to stop your car all the way first. Then turn your wheels. Then apply the gas. Etc
Does that make sense?
Basically, just use all your signals or cues for your horse to get the best end result of the manuever you want to do.