That would be the slipper clutch, a safety mechanism to save your circuit boards if the tank encounters excess resistance (trying to push something heavy, something caught in sprocket etc), not enabling the motor to "stall" (max amp draw/possible board damage). You can [link=http://members.iinet.net.au/~wackywheelz/]disable[/link] it, allowing the plastic gearbox to act like a silent metal gearbox, never giving up in the toughest of terrain, but then you loose the safety mechanism.

Swathy, if only the Bulldog looked good. Everytime I look at mine I shudder, its just so damn ugly