A state of maximum activity, energy, or force. For example, His mind was in high gear as he studied for the medical exam, or The political campaign is finally moving into high gear. This expression alludes to the high gear of an engine transmission, used at the fastest speeds.
Examples from the Web for high gear
There was a scramble about the motor-car, a frantic cranking-up and jumping start on the high-gear.Motor Matt; or, The King of the Wheel
Stanley R. Matthews