Why does my car jump when I turn it off?

Contrary to popular opinion, modern rear-wheel-drive cars are much more user-friendly in regard to oversteer. Their suspensions are balanced heavily toward understeer so that they are largely incapable of oversteering even if the driver attempts it on purpose. More powerful cars typically have on-board computer systems which can automatically brake the wheels or override the driver's throttle inputs to prevent an oversteer condition (whether the driver wants it or not). This is because understeer is generally much safer for novice drivers, whereas oversteer is much more difficult to correct when one is not prepared for it. Many sports cars allow these systems to operate in more liberal modes or turned off completely by experienced drivers. Often this is performed by a special key sequence or other little known means so that inexperienced drivers will hopefully never try it.