The bat is held at the small end and struck soundly with the mallet at the bigger end. A yellow marker marks the location of the center of percussion. When the bat is struck below the center of percussion it will spin out of the holder's fingers moving in the opposite direction to that of the incoming mallet. When the bat is struck above the center of percussion it will spin out of the holder's fingers moving the same direction as the incoming mallet. When the bat is struck at the center of percussion it will rotate about the holder's fingers, but will not spin out of the fingers.
This illustrates how a ball player wants to hit the baseball to get the greatest momentum transfer to the ball with the least reaction force on the batter's hands and arms. A tennnis stroke works the same way. You can minimize "tennis elbow" by hitting the ball at the center of percussion of the racket so that the rotational reaction on your wrist and elbow is minimized.
Click your mouse here to see the collision of a baseball and a softball with composite bats, taken by a slow motion camera.
Click your mouse here to see the vibrational modes of a baseball bat.