On most trees now, both top yellows come on together and then on the fast car's side, there is a delay before the second yellow comes on and the tree continues down. This eliminates the crossover and arithmetic needed to figure the number to use. Essentially, it turns a full tree into a pro tree with both drivers reacting to the flash.

A delay box is electrical and requires use of a transbrake. Release the button on the flash, the box delays release of the brake a preset amount of time. The ET difference if you are the faster car, half a second for the second and third bulbs on a .500 tree, and whatever fine adjustment needed for car and driver reaction times before the tire clears the beam.