i dont run nitrous but Im sure you hook the purge solenoid up to a switch, hit the switch the soleoid opens releasing the nitous in the line into the atmosphere.
I saw this Corvette at Atco on Thursday. The guy had a 150 shot running low 11s, hoping for 10s. He had his switch in the engine bay mounted on one of the plastic engine covers. I thought it was kind of an odd place to put the switch. Anyways, the switch was a momentary switch. Press it and it purges, let go and it stops purging.
my purge solinoid it mounted side by side next to my N2O solinoid.. unlike the picture. Your "out" on the n2o is on the right side. I can take some pictures if u want.