Emerling, you're a nit-picker!! I'd pay to see a lefty try to pitch while his left leg was in the air!!
Seriously, the question has been answered, with one amplification, if I may add it. Lefty pitcher's have been given a bit of latitude on this move, where they actually stop and hold their "Right" leg up, attempting to get the runner to make a move, so they can then step toward 1st and pick off the runner. This is a BALK! Don't let those sneaky southpaws get away with this.
Bob P.