If i remember my P of F its to do with retreating and advancing blades, max flap up/down etc. These all happen at specific points on the rotation of a blade. If wind is blowing from the wrong direction i.e. anywhere but the front, where helicopters are designed to accept wind from then it puts things out of position from the norm.
Not very scientific but its all i can remeber from the top of my head.
Next thing, tail rotors (when using the pedals) do use up quite a lot of power regardless of the aircraft. The deciding factor is the wind speed and direction, just the same as the main rotor problems.
Sorry but thats all i can offer just now because my notes are at work and when i get to work i will research it and give a sciency type answer
