Refer to figures.
Best Rate of Climb Airspeed (V_y) is the airspeed at which the helicopter has the biggest difference between power required and power available and gives the maximum altitude gained in the shortest time. The greatest difference occurs at the lowest point of the power required curve, where the corresponding speed is the Minimum Power Airspeed (V_MP) and Maximum Endurance Airspeed (V_{MAX END}).

When turning, the total rotor thrust must be increased raising the collective, if its vertical component equal to weight is to be maintained keeping constant altitude. The greater the angle of bank, the greater the total rotor thrust and, thus, power required, an effect similar to that of increased weight.

Since total rotor thrust required increases during a level turn, so induced drag increases. Parasite drag remains almost the same, so the total drag and, thus, total power required (Total Drag x TAS) increase more for low speeds (such as V_Y), than it does for high speeds. This effect causes the power required curve to move up and slightly to the right, as shown in the figure.

As a result, the lowest point of the power required curve, which corresponds to V_Y, moves to the right and increases.