Tartare - ETL is broadly in the range 12-20 kts - it is difficult to know exactly when flying since the airspeed indicators are poor at low speeds.
As someone mentioned before, the rotor without ETL has to work hard to suck all the air through it and accelerate it to produce thrust = high power requirement.
Once moving forward (or in any direction) at sufficient speed, the air approaching the rotor has a horizontal component that changes the inflow angle, increasing the AoA and giving more thrust for the same collective setting. The aircraft wants to climb so we normally let it - the whoosh!
A number of other aerodynamic effects are also noticeable just before ETL (flapback/blowback and inflow roll/transverse flow) but these are easily corrected for by the pilot. You often get pitch disturbances above ETL on helicopters with large horizontal stabilisers as the airflow changes during transition to forward flight - the AS365 had a marked pitch nose down at about 30 kts.
Managing all of this whilst still looking good and talking cool on the radio is clearly why helicopter pilots are so superior
