In either case, it is the lynchpin. A small object controls much larger vessels. Three illustrations of its expansive capacity are given. First, in v. 3 the tongue is compared to how a bit changes the direction of a horse. It is, by comparison to humans, a large beast.
Horses also have to be tamed.
Once tamed, they may be steered, but their natural tendencies still have to be have to be reined in. People do that by saddling and bridling a horse. The reins are connected to the bridle, and specifically a bit is placed in the mouth of the horse, over his tongue. If the rider pulls up on the bit, the head of the horse raises, and he slows down. The tugging of the small bit, one-thousandth of the weight of the horse, can bring a large stallion to a halt.