ETL = 'Effective Translational Lift' ; a term from across the pond I understand.
As for the shudder, i thought it was caused by vortex roll-under. Hence the shudder is greater when closeer to the ground but tends to be far less when flying a climbing transition.
Doesn't explain the shudder (and god didn't the ol' Wessex shudder!) when returning to the hover.
J