In the still-air hover, all the airflow is being sucked from above the rotor and blown downwards. The column of descending air is called Induced Flow, and it reduces the angle of attack, making the pilot use more collective pitch to compensate.
With forward movement, or with a breeze, the air at the front of the disc is seeing airflow coming at it more horizontally instead of all vertically, that is, not induced flow, so the front gets a higher angle of attack and more lift. (This causes other effects such as flapback and further complications like inflow roll, but that's another topic.) As the forward speed increases, more of the disc sees the airflow coming more horizontally than vertically, though at the back there is still a large vertical component - you can't suck the air down without the air above it coming down to fill the gap.
Overall, though, somewhere between 12 and 18 knots is the "leap into the air" point, the sound of the rotors changes with the change in AoA, and the pilot smiles and gets hard.