when a motor is spun by it's shaft, and instead of power being applied at the motor terminals, the power is dissipated in a load, the motor acts as a generator, and as power is dissipated in a load, it creates a back-torque in the armature which brakes the shaft. you can see this action for yourself with a stepper motor. with the drive wires disconnected, as you spin the shaft, you will see an AC voltage appear across the windings, the motor is acting as a generator, and with no load is easy to turn. if you tie all of the wires together electrically, and now try to spin the motor, it's difficult, because most of the power you are applying is being dissipated in the low resistance of the windings and the connecting wires.