One way is that the drag racers choose a time at which to go at. On the top right of every lobby there is a clock that shows how long the lobby has been open so if the time was something like 12:34:XX, the racers would say '40' or '45' which means they will go or drag at 12:40 or 12:45. This way is perfectly fine but it is very easy for someone to jump the start and get an unfair advantage.
Another way is by 'ghosting'. Ghosting is when another car sits a few yards or so ahead of the draggers and presses the 'Start' button which makes their car go transparent. The car going transparent is used as a 'Green Light' for the cars to go. Only the person who is ghosting knows when he will do so, and that makes it harder for people to jump the gun.
Hope that made sense and hope that it helped.
