On a runway that is slated for "departure only" (e.g. 18L or 17R) at a particular time, ATC will follow this pattern (aircraft departed in numerical order):
AC1, winds @yy, fly heading 170, runway 18L clear for t/o.
AC2, runway 18L, position and hold
AC0, contact departure
AC2, winds @yy, fly heading 190, runway 18L clear for t/o.
AC4, runway 18L, position and hold
AC1, contact departure
AC4, winds @yy, fly heading 165, runway 18L clear for t/o.
AC5, runway 18L, position and hold
AC2, contact departure
Notice the flow. By the time AC1 was off the ground, AC2 was pulling onto the runway and the prior departure (AC0) was passing through an altitude where it was to be handed off to departure (~2000-4000 AGL)
The only time you will hear a break in this at DFW
is when they will tell a positioning aircraft that someone will be crossing downfield - an arrival off of 18R. As soon as the aircraft that is departing passes where the arrival is holding on a taxiway, they will clear the aicraft to cross... and THEN (when the crossing AC
is clear) give the t/o clearance to the one in position. Usually, it doesn't matter because the a/c that is positioning only just gets into position by the time the arrival has crossed. At times, you will see them hold 2 or 3 aircraft on separate taxiways next to the departure runway and then have them ALL
cross behind a departure! That's kinda cool...