The horse is trained to move away from pressure. As the rein is laid on his neck, he moves away from the rein. It came about as the need for controlling a horse using only one hand - as in warfare and as cowboys working a herd. A neck reined horse is normally ridden on a loose rein since it isn't necessary to maintain contact with his mouth.
It is very useful to teach all horses including English ridden. There are many times when it is just convenient to be able to ride with just one hand.
Most horses I know that neck rein don't need the leg ques or anything.
I have a horse that you barly even have to touch her neck with the rein and she turns, depends on how much pressure you put on her neck to how much of a turn.
When I am showing someone how to ride my horse (as in a visiting friend for a 10 min ride in the corral, not a lesson), I find people can get confused about neck reining.
So I just tell them to use their rein hand and point in the direction they want to go. Works every time! Lol