the lift is caused by the car acting like an aircraft wing, as you approach take off speed the pressure over the top of the car is much less than underneath it, (hence why you also have a big dirty tea tray on the back , to try and keep the thing on the floor ) and the sunroof is forced up by the higher pressure underneath it, and hence then catching the air and foooking off backwards , my saff one "pops" at about 120MPH, I really ought to sort it before that happens to me