Mopar racer Nick Bowman doubles up

I drive stick cars so pardon the stupidity here.

If I have this correct, the “box” delays the trans brake from releasing. What I can’t figure is, at some point you still have to have the timing down to get the box to release when you want it to. So if you tell the box to delay xx, there is still a reaction time no? How does this help? Why not just release the trans brake on the third yellow?

Again, just learning.