The caster is constantly teleported a few feet in a random location every few seconds, making the caster harder to hit in combat. Before making an attack roll, would be attackers must roll a d20 to determine if they can hit the caster. If the result of that roll is 1-10, that attack roll can be made. If the result was 11-20, the caster cannot be hit that round.
This effect operates much like the spell mirror image.