Ball in the Circle

Question
Bases empty.  Ground ball hit to short stop who overthrows first base.  As runner advances toward second, right fielder fields ball and throws to pitcher who is in the circle.  Runner is at least 2 steps shy of second when ball gets to pitcher.  Runner rounds second, takes three steps toward third, stops and goes back to second.  No play was made on the runner.

I appeal that runner should be out because the ball was in the circle before the runner reached second.  Umpire says that there is a "momentum rule" and runner is not out.

Questions:  Is the runner out or safe?  Why?  Is there a momentum rule?

Answer
Hi Kyle,

A pitcher cannot stop a runner legally advancing simply by having the ball in the circle. If she had stopped at 2nd and then left that would be a violation of the LBR.  Otherwise w/ your facts no problem.

Is there a momentum rule?  Not that I know of but there are LB rules.

Mark