http://en.wikipedia.org/wiki/Adverse_yaw
Assume we are turning aircraft to the right.
Rolling an aircraft right, causes phenomenon known as adverse yaw of aircraft.
Cause is the disturbance in lift forces on both wings, also the rolling of wings.
Adverse yaw means that the nose of the aircraft keeps wanting to yaw towards
the opposite direction of the roll.
This is bad because we want to turn to the same direction as the roll. (obviously)
Adverse yaw is bad news for turn rate, turn radius, and speed, it sounds like that to me.
We use rudder to counteract the effects of adverse yaw. And by using rudder right, when turning right, we are counteracting the adverse yaw because the rudder turns the nose to the same direction to the inside of the "curve", instead of nose being outside the curve.