Refer to figures.
When a helicopter is in a still air hover the airflow over the rotor is moving vertically downwards, giving a higher induced flow. As the helicopter transitions forward the airflow over the rotor is no longer moving vertically downwards over the rotor but moving at an angle over the rotor causing a reduction in induced flow.

During the forward transition the induced flow decreases and the relative air flow angle decreases, giving an increase in angle of attack leading to a higher thrust and less power is required. This phenomenon is known as translational lift.

Before reaching translational lift from the hover a small increase in power is needed to maintain height above the ground since the helicopter is moving from the ground cushion effect for Hover In Ground Effect. As the speed increases and translational lift develops the helicopter's performance improves due to increase in main rotor efficiency, the speed for development of translational lift is 10-20 kt.

Results of translation lift:

• Increased lift.
• Tail rotor becomes more efficient.
• Nose pitches up ( Flap back).
• Helicopter rolls to the advancing side (Inflow roll).