Exactly,slack is the space give to each car if the train to move before the next car has to. Starting cars rolling is the most energy expenssive thing the motive power has to do. So once you take the slack out on a single car it's pretty cheap to keep it rolling (Issac Newton to the rescue!) - then on to the next car. So a train starts rolling one car at a time as the slack gets taken out. Crazy efficent.