Sometimes the throw is so off line that the first baseman must leave first base to catch the ball. Many times just catching the ball will be difficult enough, but in some cases the first baseman may still be able to get the out.
If you're pulled off the base towards home, you may be able to catch the ball and make the tag on the runner as he goes by. When making this play remember the following:
It would be great if you always received the ball chest high from the other fielders on a throw to first. Unfortunately there are many times when the ball hits the dirt before it hits your glove. As a first baseman, this play has the greatest impact on how coaches, teammates, and fans judge your defensive ability. While you won't have an error under your name if you can't come up with the ball in the dirt, it still reflects on your ability. Although you won't field all these "dirt" balls cleanly, it is a skill you should master to the best of your ability.
The following is a good video on the correct positioning, footwork and how to pick a ball that is thrown in the dirt.
When fielding a throw that is going to hit the ground, you will be forced to make the best of a difficult play. When fielding a ground ball you can move up or back to keep from fielding the ball on a short hop. When fielding a thrown ball you are limited in your ability to move since you are trying to keep your foot on the base.
As the ball comes toward you, quickly determine where it will hit the dirt. If you cannot stretch out far enough to catch it in the air, stretch out toward the ball or stride out at an angle that puts your glove in a position to catch the ball just after it hits the ground. The closer you can get to catching the ball as it hits the ground, the easier it will be to catch.
As the ball hits the dirt, move your glove forward and at a downward angle toward the ball. This allows you to catch the ball right after it hits the ground and most importantly keep it in your glove. When catching the ball in this manner, you don't want to stretch out and then have your glove give or have it angled up. This most likely will result in the ball bouncing off your glove.
The video above explains how to catch the ball as you are partially stretched out toward the throw. This is important as it can be difficult to know exactly where a ball is going to hit when the ball is in the air. If you cannot stretch out in order to catch the ball on a short hop, then modify your approach. Instead of stretching out and trying to catch the ball just after it hits the ground, you will want to stay close to the base. This will give you extra space after the ball has bounced before you have to catch it. In this case footwork is very important. You want to put yourself in a good position to field the ball and at least knock it down. Try to position your body in front of the throw. You are going to act like you are fielding a ground ball, with the difference being your foot is on the bag. The image above shows an example of fielding this type of throw.
If you're a right-handed first baseman and the ball is thrown to the outfield side of first base, you may want to switch your feet, placing your left foot on bag. This will allow you to get in front of the ball thrown to the outside. The same footwork is required for a left-handed first baseman catching the ball on the home plate side of first.