Why Is Corking a Bat Illegal?

So-called “corked” bats have been hollowed out and filled with a lighter material, such as cork, to disguise the modification. They are illegal because they allow batters to hit the ball further, or so the anecdotal evidence suggestions. ... The reason bats are modified in this way is to make them lighter.

What is wrong with having a corked bat?

Corking a bat causes the bat to be lighter, which in turn allows the batter to swing it more quickly. However, the reduction in weight negatively affects the velocity of the ball as it leaves the bat, effectively cancelling out the advantage gained from a quicker bat speed.

Is it illegal to cork a bat?

In professional baseball, the bat must be made from a single solid piece of wood thus the use of corked bats during games is illegal. Still, corked bats have turned up several times in major league play, most recently by Sammy Sosa.