Opening up the Super Soaker Quick Blast, I was curious about how the pressure-based valve worked. I was guessing that it worked similarly to how the Hydro Blitz's valve system worked, but wanted to be sure. After opening things up, the mechanism became rather clear.

The Super Soaker Quick Blast actually has a pressure chamber! The pressure chamber, itself, is actually spring-based. Due to how the PC is closed, I opted not to force it open for fear of not being able to put it back securely together again. However, the spring is visible through the small holes on the rear side of the PC.
As the PC fills, its plunger extends out the backside, pulling on the Max-D-style valve set-up. When the plunger extends far enough, the spring-loaded bar suddenly snaps back, pushing on the nozzle-valve pin to open up the nozzle valve and allow the PC to force water out the nozzle.
Neat stuff! More pics to come soon. Soak on!

Not that it's hard to figure out.