Once, and solely as soon as all passengers are on board and the jet bridge has been removed, pilots can ship a ‘ready message’ to ATC. This lets them know that we are ready to move and signifies that if any aircraft infront of us miss their slots, or there are slot improvements available, they’ll have the power to offer them to us. As is usually the case, the slot can drastically come forwards and suddenly be in 10 minutes time.