[merkit the grof]

 Placing a resistor in the circuit connected to the windings of the armature creates a "Load".  Energy from the electrical power produced by the `generator' (ie: motor driven by mechanical power) performs work in overcoming the load imposed by the resistor, 

 This sentence seems to imply that the resistor creates the load that applies the brakes, but as Alan has pointed out, introducing an adjustable resistor reduces braking; and, the higher the resistor value the less brakes we get. Additionally, brake adjusters using diodes do not change "the load" yet they work too.
With no resistor (or diodes), we get full brakes by dumping all the generator voltage from the positive side of the motor to ground. 
With the adjustable resistor (or the diodes) we don't dump all the voltage, that reduces the braking.


Joel