You have to have your feet in the batters box or touching the line. If you have 1 foot in the air and another one touching the line you are fine. your have to make contact with the ball before your lead foot hits the ground outside of the batters box.

your feet have to be in the batters box at the point of contact with the ball

The one thing about slappers is that it is very difficult to see where their feet actually are right at the moment of contact, as they are still moving forward when they are driving through the ball but have already made contact. To add to the difficulty for the ump is the fact that he/she is concentrating on the strike zone not necessarily the batters feet.On top of that normally within the first few batters, the front line of the batters box is wiped out so it is hard to tell where exactly their feet are in comparison to the line. Unless the player is extremely early and out of the box, you are going to have a tough time getting that call.

you need to be in the batters box at all times until you make contact becasue if you get hit by the ball after it has been hit and you are out of the batters box you can be called out.