A curb strap is piece of tack that applies pressure under the horse's chin that produces the leverage and timing of a curb bit.
The types of curb straps or chains vary. A Single chain will apply more pressure than the double chain variety. An all leather curb strap is less harsh and is more supple. A common rule of thumb to follow when adjusting curb straps is that you should be able to fit two fingers vertically between the strap and the chin groove. A looser curb strap will allow time for the horse to get the signal that pressure under its chin is coming. Riders who have quick hands or are prone to jerking would be better off using a looser chin strap. On the other hand, riders who have quiet, soft hands and great timing can get away with using a little bit tighter strap. When a rider has great timing, he will reward the horse by releasing pressure the instant his horse performs the desired action.