My turn to ask about a balk....

[tmp44]

I wasn't working this game, just watching...FED Varsity, Right handed pitcher, runner on first. In an attempt to pick off R1, F1 (in the set position, has taken his sign, and has come to a pause) spins on the rubber and throws to first. F1 never left contact with the rubber, just simply pivoted and spun, i.e., F1 did not step off the rubber and then throw or do s snap move to first...bottom line, back foot was in constant contact with the rubber during the entire pick off attempt. Balk? I would appreciate rule references if anyone can provide them. Thanks!