ETL in a helicopter can be simply put as "the helicopter rotor system is operating in clean air and is more efficient". That may be enough for the examiner at the private pilot check ride, but most students want to understand it a bit more. I made a video that helps explain why the helicopter becomes more efficient around 16-24 knots. The mechanical angle or pitch of the main rotor blades can be left alone on take off and the angle of attack will increase with foreword flight, simply because of the change in relative wind.
ETL is something you will want to recognize as you are slowing down as well! When you get below ETL, recognized by a vibration near 16-24 knots, you will want the VSI to be under 300. See "Settling with Power".
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