Electronic Stability Program
More commonly known by the generic term Electronic stability control (ESC).
(ESC) is a computerized technology that improves the safety of a vehicle's handling by detecting and preventing skids. When ESC detects loss of steering control, ESC automatically applies individual brakes to help "steer" the vehicle where the driver wants to go. Braking is automatically applied to individual wheels, such as the inner rear wheel to counter oversteer, or the outer front wheel to counter understeer. Some ESC systems also reduce engine power until control is regained.[1]
http://en.wikipedia.org/wiki/Electronic_stability_control
Starting with MY2001, all S4s come standard with ESP. A button inside the cabin "disables" ESP's throttle intervention, but not brake intervention. In order to fully disable ESP, fuse #42 must be pulled from the main fuse panel between the driver side door and the console panel. This disables power to the ESP hydraulic pump.