I beg to differ, but if you can offer some proof to prove me wrong than, by all means, present it. But if it does operate the way you describe, then why will a dart fire if the trigger is not depressed. For instance, when the bladder gets low, you get pull the trigger and watch the spring compress, then release the trigger.
I have some pictures of a cut-open PowerClip valve on the other machine which I'll upload to my photobucket later.
Until then, I will describe the valve system the best I can.
1) The trigger is pulled allowing the compressed air to pass through to the piston.
2) The compressed air pushes back on both seals which in turn compresses a spring behind the rear seal and a compression spring connected to the front-most seal.
3) The air is still flowing into the piston while the springs are compressing more and the piston mechansim grows in length.
4) The rear spring is close to full compression and the front spring is now compressed as far as it can, so the shaft now has no compression - the front seal pulls back.
5) Now the front seal has removed itself from the exhaust hole, the air rushes out the exhaust tube.
6) The pressure inside the mechanism has dropped considerably, thus allowing the rear spring to expand again, which in turn pushes the front seal back against the exhaust hole and sealing it.
7) THis cycle continues while the trigger is depressed as there is a constant air flow to the mechansim until the trigger is released, thus cutting off the air supply to the mechanism, thus stopping the cycle.
7a) The cycle will only stop when there is no-longer enough pressure in the system to compresses the rear spring.
I do believe I had just adequately described a RapidFire20's mechanism which relies on a constant source of air pressure. Springs are not compressed back to allow a plunger to compress more air and shoot the dart, but rather the springs are there to allow a regulated amount
of air through to shoot the darts.
Satisfied? If not, I will carry on and post cut-away photographs of a PowerClip's mechanism.