The first move on a pickoff attempt to any base from the rubber must be a step toward the base. If F1 lifts his pivot foot off the pitcher's plate and throws or fakes to a base BEFORE that foot hits the ground behind the rubber, we've got a balk.
This is really no different than a "jump and spin" move; except in slow motion. Could very well be that the Base Ump made the correct call.
Jerry
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