The curb group acts by applying pressure to the poll and chin groove as well as points within the mouth, to lower the horse's head and tuck the nose in by leverage. Curb bits if used by heavy-handed or novice riders can cause extreme pain to a horse by "squeezing" the head. Also a hard yank on a curb bit with a curb chain attached has been known to break the horse's mandible (jaw bone).

Curb bits (except for gags) are used with a curb chain in order to prevent the bit from moving beyond a 45degree angle , which would put an unacceptable amount of pressure on the poll, by acting as a stop using the chin groove. This is why correct fitting and measurement of the curb chain is vital when fitting a curb bit.