This electrical device provides an open or closed circuit based on the engine being between two RPM values (hence "window") that you chose, so that you'll only flow nitrous in this range. Why would you do that? Well, for two very different reasons.
1) At low RPM, think about what's going on: you're spraying nitrous into the intake at a constant flow. That is, the nitrous bottle and solenoids have no idea what RPM you're at, and they're just pushing it into the intake at a constant volume. Inside the engine, though, the nitrous and fuel combination is being sucked into the cylinders during every stroke. The net result is that at low RPM, you're getting far more of the mixture into the cylinders. At 3000 RPM, for example, you're getting twice the amount as at 6000 RPM. So, you can imagine that running nitrous at, say 1000 RPM, is far more stressful on the motor as at 3000 RPM, and typically causes a "nitrous backfire" - meaning that the nitrous/fuel combination can explode in the intake manifold (rather than the cylinders) - a bad thing. So that's why you don't want the system triggered at low RPM.
2) At high RPM, the situation is easier to explain. Given the discussion of the rev limit above, you may just want the nitrous system to cut off before hitting that rev limit. Unless your rev limit is implemented by an aftermarket ignition, you have to set your window switch RPM lower than your Rev Limiter. It's not safe for the motor to run nitrous during the rev limit, as fuel is cut off during limiting.
Fuel Pressure Safety Switch (FPPS) A Fuel Pressure Safety Switch (FPPS) is designed to protect your engine from a destructive "lean" condition caused by low fuel pressure when there is a problem at your fuel line.
On cars with EFI, a FPPS is generally pre-set around 33-35 PSI, however, they're adjustable. To adjust the set point: turn clockwise to increase the set point, counterclockwise to decrease the setting.
Ignition Timing
Timing plays a key role in the performance of a nitrous kit. Due to the increased cylinder pressures, it's unnecessary and highly inadvisable to advance the timing as much as a normally aspirated performance engine. Overly advanced timing leads to detonation and can cause severe damage to the engine's internals. Anyone who has advanced the timing too much in any engine has heard the dreaded knock and ping.
How much timing change you'll need will differ according to engine condition, bottle pressure, even weather conditions, etc.
As a general rule of thumb, 1 degree retard for every 25 HP is common.
EFI engines may not need an adjustment to the timing up to a 100hp as the ECU may automatically compensate for any minor adjustment / requirements that are needed.
For carbureted vehicles general guidelines below may be followed:
Up to 50 HP use standard timing 50 to 100 HP retard by 2 degrees 100 to 150 HP retard by 4 degrees 150 to 200 HP retard by 6 degrees 250 HP and up retard by 8 degrees
From these baseline points if no detonation is experienced gradually increase the timing until detonation occurs. At which point retard the timing by 1 degree as a safety margin.
Companies like MSD, Crane, Jacobs and Mallory make ignition systems that are designed for use with nitrous. MSD's Digital-6 Plus ignition box has a single-stage retard built in that will automatically retard the timing when the nitrous is on. MSD also offers a Digital Multi-retard that features four stages of retard. According to MSD, "Each stage is adjustable from 0-9 degrees of retard and a maximum of 20 degrees can be pulled out".
