wot switch or not the fuel still puddles and the nitrous backs up in the manifold. You can have a nitrous backfire if you need to peddle the car. Wet single fogger systems will have nitrous backfires no if's and's or but's.
This is really common on the GT40 intakes and such with the long runners. If you get off the gas your throttle plate closes and the fuel starts to puddle and then hits that hot cylinder when the intake overlaps. Thus creating a nice explosion that will blow the upper intake apart.