I had a look in the baseball rules
"8.05 If there is a runner, or runners, it is a balk when-"
"(c) The pitcher, while touching his plate, fails to step directly toward a base before throwing to that base; Requires the pitcher, while touching his plate, to step directly toward a base before throwing to that base."
this is unless the pitcher disengages from the rubber and throws (stepping off the rubber and throwing)
however in section 8.01 (b) it says:
"After assuming Set Position, any natural motion associated with his delivery of the ball to the batter commits him to the pitch without alteration or interruption."
so there you go as long as you dont start your natural pitching motion and stop to pickoff you should be fine