 the bit is derived from the old english word meaning bite and is the mouthpiece in a horse's bridle that is used to control the horse's movements when a horse is being ridden normally the reins press the bit against the soft part of the horse's mouth causing him to turn his head but if the horse grabs it so that the bit is between his teeth it takes the control away from the rider and the reins have no effect the horse can then run freely however it chooses