Adverse yaw is when an airplane twists in the opposite direction of a roll. Adverse yaw is a secondary effect of the inclination of the lift vectors on the plane's wings caused by its rolling velocity and of the application of the ailerons.
According to Mountain Flying, flight controls are balanced and responsive; however, ailerons have an operating defect. In order to turn, an airplane must be banked, so the total lift is divided into a vertical component that supports the weight of the airplane, and a horizontal component that causes the turn. The aileron control is moved left to make a left turn, while the right aileron moves down to increase the camber and lift of the right wing. This causes the plane to rise. The left aileron then moves up, decreasing the camber and lift of the left wing.
The adverse yaw problem happens because the plane's lift and drag are directly proportional. The increased lift of the right wing causes increased drag, while the decreased left on the left wing causes a decrease in drag. Under these conditions, if the plane is making a left turn, it will yaw to the right, while a right turn causes it to yaw to the left.