Pneumatic Valve Springs !

[S. McManus]

"Valve float is caused by resonance of the valve spring when it is
excited at its natural frequency or a harmonic thereof."
-Chris Howard

I always thought it was the inertia of the lifter/valve/partial mass of
the spring.  In other words, the valve follows the correct path with
respect to the cam until the region of the largest part of the cam lobe.
But after that, the respective valve train components continue to travel
(now in the wrong direction with respect to the cam) because the valve
spring isn't strong enough to resist the inertia of the components.

What is your reference for this information?

Sean McManus
University of Washington