take the bit in one's mouth

Also, take the bit between one's teeth. Throw off restraints and proceed on a headlong course, take control. For example, My partner took the bit in his mouth and bid a grand slam, or Jane took the bit between her teeth and now there's no stopping her. This idiom alludes to the bit, the metal mouthpiece of a bridle whereby a rider controls a horse. [c. 1600]