The look-back rule is triggered when the pitcher has control of the ball in the pitcher's circle and is not attempting to make a play on a runner (including a fake or threatened throw); any runner stopped on a base must stay on the base, and any runner not on a base must immediately either advance toward the next base or return to the previous base. Any subsequent change in direction or stop by the runner while off the base will result in the runner being called out, so long as the pitcher does not attempt to make a play.
Why do we have the rule? In softball, runners must stay on the bases until the ball is released from the pitcher's hand. The look-back rule forces runners onto a base so the next pitch can be thrown, and provides a means to avoid time-consuming "cat and mouse" games on the base paths that detract from regular play.
Does the pitcher have to look at the runner to trigger the rule? Interestingly enough, no, the name is a bit of a misnomer, as there is no "look back" requirement in the look back rule. So long as the pitcher has control of the ball in the circle and is not trying to make a play, runners have to advance or return, regardless of whether the pitcher is looking at them.