When students first learn to fly helicopters, one of the first things they often notice is a slight shudder that occurs when the helicopter is flying at about 15 knots. This vibration is followed by a change in flight characteristics. The nose rises and the aircraft rolls slightly right as the efficiency of the air flowing through the rotors increases.
Effective translation lift is the term for the phenomenon. At slower speeds, the air flowing into the rotors is dirty (or recycled). It is pulled into the rotor system, hits the ground, and is then pulled back in by the rotors. This leave the helicopter operating in a messy wind environment. However, as airflow becomes more streamlined with speed, the helicopter passes through effective translational lift. It will effectively outrun the dirty air and instead start receiving a stream of fresh clean airflow from the air in front of the flight path.
When pilots passes through ETL, they must add forward and left cyclic pressure. This helps the rotor to maintain a constant attitude and level flight path.