[Al Skierkiewicz]

To make this more accurate,
DC, brush type, permanent magnet, motors will act as generators when the shaft is turned. The output voltage is proportional to the speed of the shaft rotation. If a load is connected to the motor, then current will flow. In the case of the speed controller "brake" function when there is no speed command the output transistors are turned on in such a way that two of the four output banks are turned on across the motor. This produces a very low resistance, (less than an ohm, close to a short) and all the motor current is dumped into the speed controller. The current flowing produces an effect known as "back EMF" which acts against the rotation of the motor shaft and thus the braking. Trains, subway cars, and a variety of other appliances use this to slow down, a technigue known as "dynamic braking". In large motors, a bank of resistors are switched in groups to control the amount of braking. The lower the resistance the more braking. They are very easy to see on the under side of subway cars and trains.
Most teams use this feature with care, the braking can be so severe that robots with a high center of gravity can easily tip when braking.
Hope this helps.