I read in Air Clues once of a JP QFI and his stude taxying for a sortie. The student was a nervous type and to put him at ease the QFI casally asked "what is the longest piece of metal in a JP?"
The stude begins to panic and suggests the main spar, fuselage longerons, etc, etc. At this point they reach the hold and the QFI tells the stude to forget his question and get clearance for take off. This is given and the stude enters the runway. He applies full power and is mystified when the aircraft doesn't move.
He closes the throttle, looks around the cockpit with increasing despair and opens the throttle again. The aircraft still doesn't move. In desperation he turns to the QFI and says "I'm terribly sorry Sir, I don't know what's wrong".
The QFI says, "The reason we're not moving son, is that I have my feet on the brakes. I have my feet on the brakes because I fail to understand why we have six thousand feet of runway behind us and 75 feet in front of us."
The moral of the tale was don't add distractions to a student's already overloaded brain, if not strictly necessary.
The answer to the question is the winding on the commutator of the generator