OK, stipulating that this has nothing to do with transverse flow, nevertheless per your own statement you don't get flapback until the vibration clears. That would mean that during the condition of vibration it is not a fully developed state of ETL, or perhaps not a state of ETL at all, which was my original point despite the aerodynamic arguments I was referencing from various manuals. Also, the figure out of Prouty is not relevant because we were originally discussing an approach, not forward flight in ground effect.
Semantics alert!!!!
The vibration is not transverse flow - it is the boundary of ETL (not incipient, not fully developed just ETL) and is due to the vortices clearly shown in the diagram. It happens too quickly to separate in minute detail - the vibration happens and almost immediately the nose pitches up (flapback) and the aircraft climbs (ETL).
On the way down the approach you can ride the burble (vibration), as Robbie describes, and are remaining just above the boundary of what you could call non-ETL. You can feel if you lose that ETL as the burble disappears and your power required to maintain the angle of descent increases - you have lost ETL.
We weren't discussing a speed reducing descent. Robbie said he liked to ride that vibration down, so it's a constant speed descent.
see above, to get to that situation, you have to reduce speed - if you reduce speed too much you lose ETL.
Also, the figure out of Prouty is not relevant because we were originally discussing an approach, not forward flight in ground effect.
You can't ignore the aerodynamic effects of transitioning from the hover when you are considering the approach to the hover - what goes up must come down. The figure from Prouty is very relevant.
I'm not an aerodynamicist either but I do know what a helicopter does and doesn't do.