USSSSA Rule Book. Page 41. Rule 8.2 Every fastpitch rule set has what amounts to the same rule in it.where can i find this in the rule books.. i havent run across this yet
At it's core it says that once the pitcher has the ball in the pitching circle and is not making a play all baserunners must decide whether to continue to the next base or return to the previous base and can change direction ONCE. They can't stand off the base in the middle or try to draw a throw by going back and forward.
There is some nuances to the rule. The rule doesn't apply until the batter-runner reaches first. Immediate decision is a judgement call by the umpire. A play being made includes a pitcher looking to throw (so ball being held in throwing position). It HAS to be the pitcher in the circle not another fielder. And so on.
There is a lot of discussion on this site about it - just do a search.