They dont work like that. They vent excess boost from before the throttle plate when the throttle plate snaps closed (when you come off throttle). It will do that every time you close the throttle plate, regardless of the revs you were doing before it.
When you come off throttle, the throttle plate closes, at that instant, you have high pressure on one side, and the manifold instantly goes into vacuum. This vacuum is run to the dump valves vac port, the vacuum created by the engine in the manifold when you close the throttle plate is fed to the dump valve, the vacuum foruce pulls the dump valves piston upwards, opening the dump valve vent ports.
Now if this happens at 2,000 rpm, you wont even have been on boost, and it'll vent what little positive pressure was in the boost pipes, not much of a PSSSSSHT, maybe a slight hissss. Do the same at 8,000rpm, on full boost, and the dump valve will be venting 22psi of boost out the vent ports.
What I'm saying is it doesnt "not activate" at low rpms, it always vents, you just cant hear it as much when you're not venting full boost. If you had a really wierd turbo/map and only hit boost at 7,000rpm, then the dump valve would be silent venting up until 7,000rpm, lol. Basically it'll be full volume on full boost, no way can you avoid that, or set it.