One explanation I was given (and I'm no mechanic!), is that it is to do with the shaft whipping about in the bearings. At high engine RPM, the shaft will oscillate about it's axis. When the clutch is engaged, the shaft is braced thereby preventing it 'whipping' about.
In autorotation, the freewheel unit disengages the drive to the rotor transmission but the clutch remains engaged, the belts still tight and holding the engine output shaft.
This could all be wrong and those in the know will surely descend shortly. I don't believe everything I'm told although this sounded credible!
SEL