A mechanical speed control is a simple electrical device used to control the speed and direction of the drive motor on older and/or entry level electrically-operated R/C surface-operated vehicles. They use a servo-operated switch to connect the battery directly to the motor or, through a combination of ceramic resistors connected via a rotary switch, to vary the speed. The earliest types of mechanical speed contols utilized a servo-operated wiper arm making direct contact with a wire-wound resistor. These types of controllers net lower run times and slower speeds than electronic speed controls. In addition, the resistors get quite hot when running at low speeds. Should your model still be equipped at this late date with a mechanical speed control, an upgrade to an ESC is highly recommended.
The mechanical speed control is a descendent of slot car controllers, hardly surprising as many of the R/C car manufacturers started in slot cars. Ever-lower manufacturing costs of electronic controls have rendered mechanical speed controls on most new electric models obsolete. The "Expert Built" line of 1/10-scale ready-to-run cars from Tamiya still employ mechanical speed controls but are easily upgradable.