2 Possibilities.
1. The handle switch may have a mechanical brake line to the motor
2. The switch may short circuit the motor to slow it down. With the motor energized, even if it's a series universal motor, it will when spinning freely, act like a generator to a degree, short circuiting that puts a load on the motor and brings it to a stop quicker.
Unplugging the mower with the handle switch in the normal 'run' position will bypass either of these devices.
[edited to complete possible explanation]
[This message has been edited by chipmunk (edited 05-10-2005).]
