Why does your car jerk when you use the defroster or air condisher?

When the air conditioner comes on, which includes turning on the defroster at low temeprature ranges, it draws a lot of power from the engine. This can result in a decrease in idle speed and less available power on starting off from a start. This is often felt as a shaking or jerking. It is possible that the idle is low or that there are problems preventing full power.