Introducing a yearling to the bit for the first time — the sensation of a metal object in its mouth, the weight of it on the tongue and bars, and eventually the feeling of rein pressure through it — is a significant step in its training progression that should be handled with care and patience. A horse that accepts the bit willingly and without tension in its jaw and neck will be far more educable in its subsequent ridden training than one that was bitted carelessly or before it had developed sufficient jaw and neck relaxation to accept contact comfortably.
The snaffle bit is the standard choice for initial bitting, with a smooth, jointed or straight-bar mouth in an appropriate thickness for the horse's mouth conformation. Before the bit is introduced, the horse should already be comfortable with the handler's hands around its muzzle and in and around its mouth — fingers rubbing the gums and tongue, pressure on the bars — so the sensation of something in the mouth is not entirely novel.
The bit is introduced by first allowing the horse to sniff it and investigate it freely. Then, with the bridle or bit holder in place, the bit is offered to the horse's mouth with molasses, honey, or another palatable substance on it, allowing the horse to take it voluntarily rather than forcing it in. Once the horse is comfortable holding the bit in its mouth, the cheekpieces are adjusted and the horse is allowed to stand quietly with the bit in for progressively longer periods — walking in hand with the bit, ground driving with light rein contact — before any actual rein pressure is applied.
Many trainers delay introducing bit pressure until the horse has been carrying the bit quietly for several sessions, using that time to establish jaw relaxation and acceptance of the weight of the bit before asking the horse to yield to any rein cue.