This is an AC motor brake circuit. This circuit is used to make the motor stop/decelerate faster (braking), more than just cutting the power of the ac power tool motor. Here is the schematic diagram of the circuit:
The braking mechanism is done by a diode and switch S1. When this switch is on, the motor will be fed by an AC power and run the motor. If S1 is pressed down , the motor will decelerate because of DC power is fed to an AC motor. This brake circuit work only on AC motor, it does not work on universal (DC-AC) motors since DC voltage won’t stop the motor (the motor will just decrease the speed as the power is decreased by a half). To protect the diode, a heatsink must be installed properly.