How are you going to compress the fluid? If you are going to have the "tank" be flexible to allow compression, that may be considered springy. Maybe, IDK. Also the thing about that spring fatigue, If you do have this flexible/expandable tank, it will probably be made of synthetic materails. Your fluid may, break down the tank. And the tank may crack in a few years.
Unless your lever is a pump, but if you want to open the blade fast, you'll have to pump one time fairly hard? Correct? I don't really see, the point. Our current switchblades are pretty damn tough as it is, and I can't even imagine having plumbing in a knife.
What about pneumatic though, same principle, but the blade will open slower. Unless the cylinder already has some compressed air, making it's reaction time better.
Co2? Holy hell! I got an idea!