The rollerons are spun up to speed before launch using compressed air I think. During flight any roll will cause precession of the flywheels, which are attached to control surfaces on the rear of the fins. The movement of these control surfaces counter the roll of the missile. With the missile rolling less the targeting system has a lot less to think about and accuracy improves.
Roll introduces a phase shift between the orientation of the control surface and the direction is is trying to control. The faster the roll, the more phase shift. Think something like the elevators on a plane being moved, but the yaw (left/right) direction being controlled by that movement (but in a rotating frame of reference too!). Phase shift can get quite high.