WOT = Wide Open Throttle.
Window Switch = It tells the nitrous to only spray between a certain "window" of operation.
I.E "Only operate between 3000rpm and 6000rpm" or wherever you want to make that "window" start and end.
HE suggested a few hundred RPM below your rev limiter so that when your rev limiter hits and cuts off fuel, you dont blow your motor apart (as you could if you were using nitrous at that moment, because the gasoline helps cool the cylinder and without the gasoline you run insanely lean and melt ****)

So yeah, there you go.