using your foot would do the same thing
but in a 7 second ride stuff happens too quick to do that consistently
that is why they have controllers...
with one you can make very small changes and see what it does on a time slip
there are also data loggers (another puter) that records many different factors - tire slip, clutch slip, temps, boost, rpm, egt, 0x, what ever you want to hook a sensor up to. after a pass you can look at that data (and you can even lay one run on top of another) and use that data to make changes to things such as a boost controller or the clutch or maybe even tire pressure
there is more to drag racing than you could imagine
don