The "AYC" that the Evo has is linked in with its "ACD" to give it more control over what the car is doing. The yaw control computer takes in things like speed, steering angle, throttle position, yaw angle, and such to determine what the car is doing. But, more importantly, it tries to figure out what the car SHOULD be doing. Then, it will tell the ACD, or Active Center Differential, to send power front or rear to the wheel that needs it more, and the AYC will send the power left or right in the rear to get the car around the corner. Its basically just a big mess of computers and differentials that help get the car around a corner better.
Hilg