The Look Back rule is specific to softball. The rule, in the words of USA Softball (teamusa.org), “was supposed to stop players from dancing around between bases while the ball was in the Pitcher’s possession and control in the Pitcher’s Circle.” The short, paraphrased version of this rule is this: When the Pitcher A) Has control of the ball, B) Is in the Pitcher’s Circle and C) Is not making a defensive play, the Look Back Rule is in effect. This requires a Runner who is in between bases to either advance to the next base or retreat to their previous base. If the Runner is already on a base, they must stop. If the Runner does not either advance or retreat, or if they leave their base, they are out.
It sounds simple, right? It is, but here is where it can seem difficult. The confusion begins with the fact the Look Back Rule is a misnomer (see “Foul Tip” post). In actuality, there is no requirement for the Pitcher to “look back” at the Runner. It is also important to remember the Runner is entitled to read the play (i.e. are A and B and C from above met?) and decide what action to take. Note: Reading the play and deciding what to do takes, quite literally, as little as fractions of a second, so there is no reason to dither in between bases. Finally, the actions of the Pitcher come into play. Throwing towards the base, running towards the Runner or feinting (yes, even the shoulder juke) are defensive plays that eliminate the Look Back Rule.
Ultimately, the call of a Look Back Rule violation relies on the judgement of the umpire taking into account the parameters defined in the rule.