No, it hits only when you press the throttle fully open generally, although you can have it rev limited too.

So at full throttle acclerating flat out at 4000rpm you would have nitrous injected, but at 5000rpm on a fast cruise down the motorway it wouldnt be injecting any.

There is no point injecting it anywhere other than at full throttle as anywhere less than full throttle you can have more power for free just by opening the throttle further anyway.