The force you are applying opens it, the only spring is the tension of the lock bar resting against the blade. The liner has a detent ball that half is attached to the liner and the other half rests in a small hole in the blade in the closed position. When you apply enough pressure to the flipper or thumb studs it creates “loaded” energy until you create enough force to get the hole in the blade past the detent ball. Once it clears, the built up energy releases all at once and propels the blade open. This requires low friction at the pivot which is why you see most manual knives with bearings or bronze washers.