Walter
I've read a few papers on valve rotation and I thought this one was the most descriptive about the mechanism being the valve spring through the torsional rotation of the spring wire.
Since valve springs are tuned to have a natural frequency above redline to prevent valve bounce, I assumed that a valve spring designed for a low speed engine would - through either lower tension or lower natural frequency - involve valve rotation at lower speeds.
As an aside, my 1985 Alfa GTV6 gets valve bounce about 200 rpm part redine. Its an effective rev limiter and hilarious! It happens reliably if I try and eek another couple of km/h out of 3rd gear before the braking marker on the back straight at Winton.
The question is, do valve's burn because they don't rotate or do burnt valves stop rotating?
After you give me that answer, I have another question involving chickens & eggs.