Swinging the Bat for Power
http://www.exploratorium.edu/baseball/howfar.html
Many people might think that swinging the bat straight through the ball would be enough to hit the ball a decent distance off the bat. There's many more mechanics involved in the swinging process. Muscle has only a small part to play in the swinging a bat for power. There are two types of mechanics involved while swinging a bat, Linear and Rotational. Rotational mechanics are the dominant source of power in the swing. Out of the rotational mechanics come the two forces that help generate the speed and power of the swing, torque and the other comes from the energy of rotation. Speed from the energy of rotation comes from the path that your hands follow as you swing the bat. The speed generated by the circular rotation from your hands is like a ball at the end of a string, as long as your hands are moving in a circle then the ball continues to accelerate in a circle. So the bat will also move in at an increasing speed as long as your hands are following a circular path as you swing. Any foward movement of the hands or body in a straight line won't add to the overall bat speed.
Torque is another key ingredient to a good power hitter, or even a good hitter. Torque is the result of two forces being applied to an object in opposite directions. In this case the two forces are being applied from the hands and wrists onto the bat. As you start your swing, instead of moving your hands directly at the ball, you want your hands to start a rotation. The top hand begins to move backwards while the lead arm drives the end of the bat towards the ball. The torque invloved is created around the point of the bat that lies between your hands. This torque greatly accelerates the head of the bat which will have act with a greater force when (or if ) it comes into contact with the ball.
www.barspeed.com/research10.html
A good batter will use almost all of these rotational and torque energies either before or durning contact with the ball. After contact your body is relaxed and its remaining motion is smooth, this extra motion is caused by the momentum of the bat pulling the arms up and through.Your follow through is also very important, even though the ball has already been hit it is ideal that you complete your swing so that you're not thrown off balance by that extra momentum when you start to run.
So just remember these three things: while swinging keep hands moving in a circular motion. Remember to rotate your wrist as you swing. And always follow through.