For the front wheels...
With static toe in as you initially steer or are tracking strait with minor adjustments you increase slip angle on the outside tire and decrease it on the inside. This has the effect of increasing the pneumatic trail and slip angle drag on the outside tire and reducing pneumatic trail and slip angle drag on the inside which gives rise to a centering moment of a certain magnitude about the steering axis depending on the steering axis intersection with the ground i.e. scrub radius and mechanical trail. The point where the tire force acts moves rearward slightly and slip angle drag incurring on that side reduces the yaw moment generated by the lateral component of the slip angle. A lower yaw rate vs. steering angle gain and overall near center steering stability.
Another way of looking at it is that both are trying to drive towards the center of the car slightly and the forces along the steering shaft are balanced. When one is disturbed or initially steered from the central position it is pulled back by the inequality of the moments summed about the kingpins since the outside tire begins to dominate and wants to right itself. With toe out it opposite is true.
