Crab,
Does the Main Rotor have a way of knowing it is in a "Down Wind"?
Sas - that is my point - it doesn't, as you know from the Chinook you can transition sideways which gives ETL to both rotors together.
My reference to the downwind transition was purely to highlight the sequence of events such that you can have ETL facing downwind, lose it as you move forward and then gain it again when your airspeed becomes positive.
Losing the burble as you move forwards coincides with an increase in power required to maintain height and the 'smooth patch' in the low to zero airspeed condition is a marked difference from ETL at the beginning and at the positive airspeed position.
The last couple of knots before gaining ETL again is where the maximum power required is - something that often catches people out when transitioning downwind with limited power.