More rollout between the stage beam and the starting line.......it's like getting a mini headstart before the clocks begin.
Just to add: Taller tire (larger circumference) with more rollout will give a hair better et. which is why class racers (Stock,Superstock) often use them as they need everything they can get when qualifying or running heads up. However, it also slows your recorded reaction time so that must be compensated for........................On the other hand the heads up classes (Super comp, gas, street) are mainly won by reaction time so most of those racers use a short tire to enhance that part of the run. There's even more to all this if you start getting into Deep/Shallow staging, but i'll just shut up now.:D