Basicly if you're playing OBR then it has to pass the fielder within a playable distance and if no other fielder has a play then the runner is ok. If the ball is not within a playable distance then the runner would be out. In FED it only has to pass the line of fielders to make it live.
This is where the string theory comes in. Assuming not a playable ball if you run a string through all the infielders to make an imaginary wall and the ball passes past through this plane then the ball would be live. This is my understanding of the string theory and I hope a clear explanation, if not there are plenty of guys that can correct me.