Lefty's Snap-Throw

[Carl Cramer]

From the set, with both hands together, a lefty pitcher wants to snap-throw to first. Where does his left foot have to be positioned is that he has successfully 'stepped off' the rubber and can then legally begin a throw to first base?

Can he begin a throw when his left foot is simply off the rubber?
Does he have to wait until the left foot is planted?

Thank You!