They are on the verge of float because the springs are turning so they are just able to rebound and close the valve in time. They do not have enough reserve pressure to keep themselves fixed in place.
I just wanted a title and that seemed easier than "BMW engine valvetrain revving crazy fast" 
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1971 911S, 2.7RS spec MFI engine, suspension mods, lightened
Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
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