There is more to this equipment, a method of dumping excess electrical energy.
The regulations state:
With the exception of one fully charged KERS, the total amount of recoverable energy stored on the car must not exceed 300kJ. Any which may be recovered at a rate greater than 2kW must not exceed 20kJ.
The maximum power, in or out, of any KERS must not exceed 60kW.
So there is a fixed rate of power transfer, and a fixed maximum capacity of the energy storage device. But what happens if a car is in braking and KERS phase, and the storage capacity reaches the maximum of 300kJ? If you just shut off the harvesting process, suddenly there is less braking force at the rear wheels, and this could unbalance the car enough to become a problem. But what happens is that the MGU-K keeps retarding the drivetrain, and the electrical energy is dumped into a device that offload it from the car in the form of heat. That would be resistors, think of the elements of a toaster.