Take the bit between your teeth

A bit is a mouthpiece that is used to control a horse’s movements. It is normally fitted so that pressure on the reins presses the bit against the soft parts of the horse’s mouth, causing it to turn its head. This expression alludes to a horse biting on the bit and taking control away from the rider.

The earliest known use of the phrase is in John Dryden’s satirical poem The Medal, 1682:

But this new Jehu spurs the hot-mounted horse,
Instructs the beast to know his native force,
To take the bit between his teeth and fly
To the next headlong steep of anarchy.