Legal Pitch

[scyguy]

6-1-2 says ....after he starts his movement to pitch, he must continue the motion without interruption or alteration.

6-2-4d balk when ...failing to pitch to the batter in a continuous motion immediately after any movment of any part of the body such as he habitually uses in his delivery

If you deem that he stopped his motion, then we have a balk. Now since you are taking about his non-pivot leg, sometimes it is difficult whether the leg stops are is moving very slowly. Good luck