The switch for nitrous oxide that goes behind the pedal does not automatically give you a shot of No2, when your are going WOT(wide open throttle) then you can hit the "N0S!1!" but not when it is onlt partial throttle.
Why would you want power that only comes on at high rpm's? It would disturb traction and power wouldn't be linear, it would be gay. Just wire in extra cells and pull harder when you want it to go faster
EDIT: Sorry about thread jacking, some systems do not use the tps sensor but a switch mounted on the actual throttle plate where the cable actuated spool thing will engage the switch, this is the way it is on my friends chevy. But you can mount a switch anywhere you want to. Or if you want to get technical you can get a computer controlled system that will analyze you maf, tps, egt and adjust advance.