I had blogged this on my site: bluecollarbaseball.com but wanted to share here as well
Let’s start with a Righty (you are a runner on first base). When you are younger, most coaches will tell you to watch his legs (feet). That is fine for young players, however as you progress, you really have to watch the pitcher’s whole body. Sure the basic move to first will involve the pitcher moving his feet. But in order to get GREAT jumps, other factors come into play. For example, the first guy to get on base should try to get a big enough lead in order to make the pitcher throw over. This will allow the coaches and players to pick up on his move. Many times a guy will use a slow-footed (B) move, then come back over with his main (A) move (the better one). When you’re on first base, watch how the pitcher delivers the ball to the plate. Some pitchers will have a big space between their legs (like K-Rod) when they set-up, causing them to really have to lift that front leg in order to get any velocity. So when you see this type of guy, as soon as he lifts his front leg, you go. The next type of righty pitcher is called a “Leaner”. This guy usually uses a slide step, but if you watch closely, his body (shoulder) leans towards the plate first. His set up will be very straight up (almost standing directly up- legs very close). So as soon as you see him lean towards home, you’re off to steal second. The last type of righty pitcher uses a “turn” when he throws home. This means in his set position, he is slightly ‘opened’ up on an angle between home and first base. So in order for him to pitch, he must turn his front shoulder back towards home and then deliver. When you see his shoulder turn in, you go. All pitchers have keys and indicators, some just hide it better than others.
Now to the Lefty Pitchers. It can be difficult stealing on a lefty, however, these indicators will help. Firstly, if a guy goes straight up and down with his front leg, you basically have to go on first move. His leg-kick will be high, so as soon as he moves you go. Even if he picks you off, it takes time for the first baseman to throw to second. In regards to a lefty pitchers front leg, if he is a guy who really has to drive towards home plate in order to get more velocity, he will bring his lead foot behind (passed) his back leg. Once he does this, he cannot come over to first. He has to deliver the pitch. This is not very common as you move up in baseball, however guys like Billy Wagner still do it. Another lefty key is his face. Many times you will see pitchers get into the habit of being “opposite”. This means if he looks at you when he goes into his motion, he is going to throw the ball to the batter, and if he is looking at the batter when he lifts his leg, he is going to try to pick you off. This is very common. A pitcher will look at the runner to try to get him thinking, and then deliver the pitch. Watch for this tendency, because it can be very very useful. The last lefty key is complicated. You have to watch his lead foot. This happens when a pitcher goes straight up and down, as well as does not get into any habit of “opposite looking.” If you are very good and can watch his lead foot you will notice that when he lifts his leg, if his cleat is facing toes down (to the ground) he is throwing the ball home. If he lifts his lead leg and you see the bottom of his cleats (where his toe is pointed toward the sky), he is going to pick you off. It is physically impossible to point your toe down and land correctly on a pick off. The same if true when the toe is up and delivering a pitch; it can’t happen. The landing would be too awkward, so the pitcher’s cleat can tell you what he is going to do. If you can master this last one, you will be an extremely good base-stealer (if you have average or good speed).
Original Post
). It's pretty simple really 
The rules apply to both RH and LH pitcher equally. So, when a Righty lifts his leg and passes the back part of the rubber he can do 1 of 2 things...he can go home or he can continue his motion toward second in an attempt to put out or drive back a runner. He cannot throw to any other base at that point. The same must be said for a Lefty. Once his foot goes behind the rubber he has to go home or continue to second (assuming there is no pause in the motion). Like the Righty, he cannot throw to any other base. Now, if either pitchers' foot does not cross the back plane of the rubber they can attempt to put out or drive back any runner from 1st or 3rd if they step directly toward the bag. 
