Composite bats are made of many layers of fiber, resin, and glue. When a bat comes from the factory the resin and glue between the fibers are stiff and less pliable. As a is hit by a baseball or softball the resin and glue begin to break, this is what players would call “breaking in”, literally. As the resin and glue breaks up the bat becomes more flexible in that spot. The same thing is accomplished through bat rolling, it is compressed through nylon or hard rubbers rollers and the resin breaks up leaving the area more flexible. Now when we do that around the circumference of the bat 8-12 times it becomes much more flexible. This flexibility equates to an increase in ball speed and farther distance of hit baseballs or softballs. Studies have been done and found that some bats after being rolled jump up as much as 5mph in batted ball speeds. You can figure about 8 feet per mph of ball velocity.
Now, wait a minute isn’t perpendicular rolling sufficient? Sufficient is a good word because the answer is yes, but if you want the resin broken up completely you would have to make about 8-12 more passes through perpendicularly hoping you roll the bat through the correct spots. After the bat is rolled perpendicularly, an assured way to break in the rest of the bat would be to roll it through a parallel rolling machine. This breaks up the resin that the perpendicular rolling missed, for a more consistent and flexible bat. The parallel rollers can not be too long, although, because there will not be a consistent and even pressure over the length of the rollers. The rollers really need to be a maximum of a foot in length; this would cover any sweet spot on any bat.
It would seem like breaking up the resin would decrease the longevity of a bat. To an extent this statement is correct so you could spend about 24 hours of batting practice breaking in every spot on your bat, hoping you did not miss a spot or get the stick rolled. In the first case you would have about an extra 24 hours of bat life as long as you didn’t hit the bat in the exact same spot 10 times therefore decreasing its life. If you had trouble following that I will explain: bat rolling breaks in the entire bat with one swoop. Breaking in a fire stick in by hitting balls endlessly causes an inconsistent break in and heightens the chances of breaking the bat sooner. Now the bat still has a strong interwoven mesh of carbon fibers throughout the entire bat which is very strong, this is what makes it possible to stay durable and flex when the brittle resin is broken up.
So let’s go over this; bat rolling increases ball speed and has a longer life than a bat broken in the same amount through normal hitting.