Pitcher Drops the Ball...

 
With bases loaded, the pitcher comes set in the proper set position. He then starts to properly steps back off the rubber with his pivot foot. Just as his pivot foot lands behind the rubber, he drops the ball. The offense wants a balk and quotes NF 6-1-4: "A pitch dropped during delivery and which crosses a foul line shall be called a ball. Otherwise, it will be called no pitch. A pitch dropped during delivery with at least one runner on base would be a balk if it does not cross a foul line."
 
Looks like a balk. What's the call?
 
Call
When the pitcher drops the ball, there are penalties (see NF 6-1-4). However, when the pitcher properly disengages from the rubber (as he did in the example below), he assumes the role of an infielder and the pitching regulations do not apply. In the example below, there is no balk, just a ball dropped by an infielder.