Originally Posted by madwi

With a progressive unit you basically are controlling how much nitrous is being sprayed through out the RPMs. It can be % of shot increasing as rpms increase or based on time. For example, with a progressive controller you can set it for 10% at 2500 rpms and then it ramps up to 100% @ 6500 rpms. Time based example would be start at 10% and then increase to full hit over x amount of time.
With a window switch it is going to spray 100% as soon as it reads the minimum rpm and spray that until it hits the upper set limit.
The route I went is in your progressive controller thread