A four-knot rope halter in thin stiff cord, tied with no metal at the knots.
The four knots sit on the noseband where they bear on the pressure points, so a cue arrives sharp and stops the moment the hand releases. Stiff cord is what makes that work — it holds its shape, transmits the ask without absorbing it, and makes leaning on the halter uncomfortable enough that a horse learns to give to pressure rather than push through. There is no hardware anywhere on it, so there is nothing to break under a horse that pulls back.
