It doesn't do that. It is the same move a righthander makes to third except rather than stepping forward, he steps backwards. At that point he throws to first. It doesn't break that part. The one thing other umpires have said is that it is two motions. He steps then throws. My argument is a lefthander does the same to first and right hander does the same to third. I believe it is just one of those we've never done it that way things. I have not found a rule that prohibits it to me so I don't call it except when they mess up and swing their foot toward home while stepping backwards.