At what point would you step up to ferrea hollow stem valves

Somebody would have to explain to me how valve weight affects spring pressure. It's the lobe lift that dictates spring pressure. At least that's how I see it......maybe I'm misunderstooken.
Initially the cam moves the valve, but after it starts moving you need spring to control the inertia and control bounce. Lighter valves
Have less inertia, kinda like lighter weight pistons are easier on your rods and bolts.