that marks the onset of ETL and as the vibration clears the aircraft pitches nose up and climbs due to flapback and the increase in lift.
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.
If so, what causes the change in inflow angle in a steady, speed reducing descent when the disc attitude doesn't change and the inflow angles are much the same (but from underneath the disc).
We weren't discussing a speed reducing descent. Robbie said he liked to ride that vibration down, so it's a constant speed descent.
The vibration is the boundary of ETL - there isn't 'fully developed ETL' it is either there or it isn't.
But you stated above that the vibration occurred before flapback?