BobDDuck
Island Bus Driver
I think it's a time distance thing. They, cab controllers, know for the most part how long it takes an aircraft to start rolling from the hold short line. That mixed with an aircraft hurling itself down the runway gives enough separation. Now I could be totally wrong so don't take this as law.
I think technically the other aircraft has to be air born and, for Cat III airplanes there needs to be a minimum of 6000 feet of separation. That said, (like you said) it's pretty common to get a takeoff clearance when the other get is still prior to rotation.