Credit to hqdby1 for proposing the idea to me.
Force parallel to the stroke plane on a flapping wing can be induced by offsetting the static angle of the wings (the angle of attack which they hold when not moving).
This produces more drag during one stroke (as the angle of attack will be higher) and less during the other stroke (as the angle of attack will be lower). By offsetting the angle of each wing in opposite directions, torque can be induced in the yaw axis.
The net lift from the wings will stay the same as the lack of lift during the low angle of attack stroke is compensated during the high angle of attack stroke. One drawback though is uneven lift which produces roll vibration. Another is a roll moment which acts in opposite direction to that of the offset, this creates an effect similar to adverse yaw in an airplane, to compensate for this the amplitude of each wing is varied slightly.