4
Second base has been stolen without a throw from the catcher to second
The runner at third must be careful to watch the catcher
If he throws to second immediately, he must be ready to steal home
If he throws to third, he must be under control enough to get back to the back safely.
It is really important for the hitter to square as soon as the pitcher begins his motion.
It is also really important for the hitter to try to block the vision of the catcher by squaring over the plate and pulling back at the last possible moment.
This play works best with a left-handed hitter up because they naturally block the runner at first.