Steve76 - if you fly an academic downwind transition from the hover (say with 15 kts of wind) it neatly divides into 3 sections:
1. Having noted your hover power, as you move forward after you adopt an accelerative disc attitude, the power required to maintain height increases.
2. As your groundspeed reaches the same as the windspeed (ie the helo is in a zero relative wind condition) everything feels very smooth (no vibration or burble) and your power required to maintain height is at a maximum.
3. You achieve ETL with its associated burble the aircraft wants to climb. At normal climbout speed/power your angle of climb is shallower and your groundspeed is much higher than your airspeed.
I have seen many pilots overtorque or inadvertantly touchdown trying to fly a downwind departure with not enough power to do it.