get the bit between one's teeth

get / take the bit between one's teeth (throw off restraints and proceed on a headlong course, take control) — закусить удила, не знать удержу

Example: Jane took the bit between her teeth and now there's no stopping her.

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This idiom alludes to the bit, the metal mouthpiece of a bridle whereby a rider controls a horse.

see also
[вожжа под хвост попала]
[like a bear with a sore head]