have the bit between (one's) teeth

To do something decisively and with resolve. A reference to a horse that is no longer under the rider's control because the bit (part of the apparatus used to guide it) is out of place in its mouth. Once Sophie had the bit between her teeth, she became the best student in my class. We've got to have the bit between our teeth if we're going to pull our company through this crisis. Once my clients had the bit between their teeth, they went public with their accusations—ignoring my counsel, might I add.