A brake (100) of a concrete finishing trowel (10) responds automatically to the cessation of drive torque delivery to the rotor to actively brake the trowel's drive train, hence inhibiting or preventing frame rotation. The brake (100) preferably takes advantages of inherent characteristics of an inclined gear (82), such as a worm of a gearbox, to permit an internal component of the gearbox to shift automatically upon the cessation of drive torque therethrough from a brake released position to a brake engaged position. In the case of a worm gear-based gearbox (40), the worm (80) is configured such that reaction forces that are normally generated by the delivery of drive torque therethrough shift the worm to release the brake during normal trowel operation. These forces are eliminated in the absence of drive torque transfer, permitting the worm to shift to a brake engaged position.