Q: When the ball is returned to the pitcher on the mound, what determines whether a runner off base may attempt to advance or must return to their last occupied base? Is there a “halfway” rule?
A: There is not a “halfway” rule, nor any rule requiring that a runner be a certain distance from their next base in order to legally proceed. However, there is a requirement that in order to advance, the runner must be in “forward motion” at the time the pitcher receives the ball on the mound (15.01). Therefore, a runner who is running may keep running, but a runner who is stopped must go back. A runner may not pause a step off the base and attempt a late dash once the pitcher’s back is turned. Note that, as always, any subsequent attempted play that brings the ball off the mound or out of the pitcher’s possession WILL allow any runner to resume advancing at their own risk.