Short version: Once the ball is in pitcher's circle, runners must immediately commit to a base. Base can be forwards or backwards, just no standing there or "dancing".
My DD is a USA softball umpire (umpires all week long and in tournaments every weekend all over). I can't tell you how many outs that she has called over this rule including ejecting two 12U coaches out of a game and the team forfeiting the game after they wouldn't leave. EmToTheDee explained it perfectly - the only thing that I would add is that if the pitcher is in the circle and is engaging the runner (faking a throw, juking at her) then the look back rule doesn't apply.