The resistors act as a load to absorb the power from an AC motor that is acting as a generator. Assume that the frequency of the power applied to an AC motor is reduced at a rate that requires the motor speed to decrease at a faster rate than it would slow down if power were to be removed. In order to force the load to turn at the slower speed demanded by the frequency reduction, the motor produces negative torque, torque that is applied as a braking force rather than a driving force. During braking, the load is driving the motor and the motor acts as a generator. The motor produces a voltage that exceeds the voltage produced by the AC drive. That voltage would tend to charge the capacitors in the drive to an excessive voltage. Circuitry in the drive detects the voltage increase and/or the current reversal and connects braking resistors to draw a compensating current and dissipate the energy produced by the motor.
The resistors dissipate power to slow down whatever is producing the voltage that the motion generates.
