This nozzle is for the PolarStar Fusion Engine. This nozzle allows you to regulate your maximum FPS output based on your barrel length.
The following is a diagram showing the approximate FPS output of the red nozzle corresponding to each of the following barrel lengths with .20g BBs at max input pressure 120PSI:
275mm will yield ~500 FPS
375mm will yield ~550 FPS
509mm will yield ~590 FPS