I am not the most knowledgeable when it comes to auto transmissions, but this is your transmission keeping you from moving backwards. I believe the tranny at a stop in not actually disengaged in the sense that a manual would be. The engine and tranny are working on pushing the car forward, even at a complete stop, kind of like a boat prop at a very low speed. This is why you have to apply brake to all Auto trans cars at a stop or else the car will move forward. Floor it and the transmission fluid will "Grip" and drive the car forward.
Maybe someone with a more thourogh understanding can elaborate?
[This message has been edited by roas (edited 06-11-2000).]