Neck reining is not the same as directing a horse via leg and seat. Leg and seat can be irrelevant to neck reining, as it is when I neck rein. I simply move one hand forward 6 inches, so the rein will be against a different spot on the neck than normal, then left or right. I do not use leg cues. My seat? I feel free to cue him to go left while twisted around right to look behind me.
It is not bit specific. Once taught, it works bitless, with a snaffle, with a one piece curb, with a Billy Allen curb...it doesn't depend on the bit or my seat.
It is a CUE: a standard way of asking the horse to respond in a way we train him to respond. I like to move my hand forward because that is how the Cavalry taught it:
"The rein, to be effective, should bear against the right side of the upper half of the neck
, as this part of the neck is more sensitive to the rein than that near the shoulders....By using the left opening rein
in early training, and later combining the right bearing rein
with it, obedience to the right bearing rein
[Note: Neck Rein] alone is easily taught." italics in the text American Military Horsemanship
Moving the hand forward first, in my experience, also forms what the military would call a preparatory command - a command to get ready, you are about to be asked to do something. Then the command of execution - turn now - is merely a left or right movement.
Of course, like many things in horses, it can be as simple or as complex as you wish. I like simple cues and find my horses interpret them in context of the situation. Others like more detailed or more subtle cues. It depends on you, your horse, your goals, etc.