The same way a brake cylinder works. The spring keeps pressure on the cup (against the fluid, and drum brakes have a residual valve to keep slight pressure in the line,) keeping it flared out. As opposed to disc calipers - they use square O rings, they don't need something to keep them expanded.
No residual valve in the clutch, but gravity creates the little bit of residual pressure needed.
The clutch slaves have a weak internal spring - when it fails air gets in, just as the complaint says. The external spring pushes the fork, and thusly the piston, back enough.
Plus without the spring, the fork is still touching the fingers - no problem with that really (except for TO bearing wear,) but the vibration from it helps air get in too.
It's not the best cure, but it works - try it!
The right way - replace the slave - but that's alot of work!