Casting is often a misunderstood concept. While it's often true that casting is swinging with a straight front arm, it's not the whole story. What actually causes the straight front arm is the top hand going away from the body over the plate, and wrists snapping too early as shown in Figure 1 below. This initial movement outwards is the beginning to a very inefficient swing path.
b)
Figure 1: Side a) and front b) view of casting. As the top hand starts to go away from the body, it causes the front arm to go straight and the wrists to snap too early. Click to enlarge images.
Snapping your wrists too early, that is before passing the body's centre of gravity, also causes the bat head to move away from the body. Like a boat pulling a water skier, when the boat turns, the skier keeps going in the same direction he was going, restricted only by the rope. The bat head is the skier, wrists and arms are the boat. Now imagine the skier is massive, as he keeps going in the direction he was going, his momentum pulls on the rope and the boat (wrists and arms). This force pulling the boat is called centrifugal force. Centrifugal force is the outward force associated with rotation, the equal and opposite of centripetal force, as shown in the schematic in Figure 2. You've most likely felt this force taking a tight turn in the car, or on the tea cups at the amusement park. This force created by the bat head, pulls the hands away from the body, creating a circular swing. So in general, younger players who have less arm and wrist strength are more likely to cast, because the bat is too heavy relative to their size.
Figure 2: Ball is swung around on a string. Centripetal force is acting on the ball by the string. Click to enlarge image.
Hitters who cast are also more likely to roll over their wrists and ground out. To keep a nice smooth bat path the top hand must stay facing up while rotating (snapping). When the top hand rolls over, the bat head goes up making a hump in the hitter's bat path as shown here. This hump is not very efficient as it doesn't let the hitter's hands reach out as far as possible towards the pitcher.
Note: Although the video is in slow motion and an over exaggeration it is still true! Trust me, if you've ever seen someone smacking themselves in the shoulder on their follow through, this is it. This hitter is subject to many groundballs, and a bruised shoulder.
To avoid casting, the hitter must make sure to load their hands, and slot the elbow close to the body, as shown in Figure 3. Then, hold the wrists in a cocked back position until they pass the centre of gravity of the hitter. Notice that in this position would be ideal for a check swing, because the bat is not over the plate.
a)
b)
Figure 3: Front view of check swing, and before hitter snaps her wrists a). b) shows when the hitter snaps her wrists, notice her hands have passed her centre of gravity and are in front of her body.
A good drill is to hit off a tee, but place another plate in front of your front foot, as shown in Figure 4. The extra plate is a reference as to where you should snap your wrists for an outside, middle or inside pitch. Once your hands pass over the outside part of the plate, snap your wrists for the outside pitch, middle for middle, inside for inside as shown below.
To avoid casting, the hitter must make sure to load their hands, and slot the elbow close to the body, as shown in Figure 3. Then, hold the wrists in a cocked back position until they pass the centre of gravity of the hitter. Notice that in this position would be ideal for a check swing, because the bat is not over the plate.
a)
b)
Figure 3: Front view of check swing, and before hitter snaps her wrists a). b) shows when the hitter snaps her wrists, notice her hands have passed her centre of gravity and are in front of her body.
A good drill is to hit off a tee, but place another plate in front of your front foot, as shown in Figure 4. The extra plate is a reference as to where you should snap your wrists for an outside, middle or inside pitch. Once your hands pass over the outside part of the plate, snap your wrists for the outside pitch, middle for middle, inside for inside as shown below.
a)
b)
c)
Figure 4: Two plate drill. The second plate in front of the hitter's foot is divided into thirds and represents when the hitter should snap her wrists, a) outside, b) middle and c) inside. Click to enlarge images.
Another drill is to set up a two tee points, one in the middle and one 6-8 inches outside. The outside point is set up higher, while the middle one is lower with the ball set up on it. If the hitter casts, they will hit the outside tee. If they keep their hands in close to their body, they should hit the ball with no problem as seen here.
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