[AtlUmpSteve]

This situation has been discussed on this and other boards before. I don't believe anyone has ever been able to get a verified rules reference specific to this situation; so everyone answers based on what they have been told, or how they think it should be ruled.

I would reference every other game situation I can think of for equivalent rulings. If a batter contacts the ground with one foot completely out of the batter's box and contacts the ball with the bat, the batter is "out of the batter's box". If a batter-runner contacts the ground with one foot completely out of the running lane, the batter-runner is out of the running lane. If a pitcher contacts the ground with one foot completely out of the pitching circle, the pitcher is out of the circle with regard to the lookback rule. If a fielder steps with one foot completely into dead ball territory, the fielder is in dead ball territory.

Conversely, I cannot think of any situation where a player steps completely into an area with one foot where that player does not establish position in that new area. There may be one, but it isn't coming to me right now.

My conclusion is that stepping with one foot completely out of the batter's box is out of the batter's box; no matter what rule you reference. JMO.