Speed control deactivation switch (brake).
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2008 Jaguar S-Type, 2007 Jaguar S-Type, 2006 Jaguar S-Type, 2005 Jaguar S-Type, and 2004 Jaguar S-Type. However, it is still accessible from the selected car via links, so may be relevant.
If the brake pedal is activated while the speed control system is active, the ECM receives a signal from the speed control deactivator switch. This will cancel speed control and allow the throttle blade to return to the position as demanded by the accelerator pedal.