The longer the shank on the sides of the mechanical hackamore, the more leverage that is generated on the horse's nose, the sides of her face, under her chin, and on her poll. The hackamore uses a curb chain or curb strap under the horse's chin to help with the leverage. due to the mechanics of the bridle, when pulled, the bridle creates pressure on the horse's poll.
The wider the nose piece, the lighter the pressure it creates, similarly- the thin or rope nose pieces can create a large amount of pressure.
The shanks on either side of the nose piece add pressure to assist in "pushing" the horse's nose in the direction the rider is neck-reining. Trying to direct-rein with a mechanical hackamore will push and pull at the same time, in opposite directions, which will confuse the horse.