The pressures are determined by the roller operator or he gets his maximum pressures from a bat rolling guide. Once the bat has been rolled around and through the rollers the composite fibers are now loosened and more flexible. This increased flex will cause a trampoline effect propelling a batted ball further. Stretches and breaking up the resin fibers will also lengthen the sweet spot of a bat. Here are the 3 big questions I hear about bat rolling the most.
There have been quite a few unscientific studies on YouTube other various websites. All seem to agree on the numbers of 20-40 feet increase in batted ball distance. I do concur on these numbers but I have seen some bats in the past increase distances up to 50ft. There has only been one scientific study on bat rolling and that was done in 2005 by Daniel Russell PhD. His data showed bat rolling produced 20-35 feet more distance than a stock unaltered bat under the same circumstances. Each composite bat is manufactured different and therefore each one responds differently to bat rolling. Another factor when measuring the increase in distance would be the size of the barrel and length of the barrel. A shorter barrel, when broken in, will produce a smaller amount of distance than a longer barrel because it will flex more. A bigger barrel will also flex more than a smaller barrel. The bottom line is that bat rolling works.
I am unaware of any detection device or tool at this time. The only thing that comes close to a detection device is a compression tester. They measure the pounds of pressure per square inch and associations have set limits on the allowable pressure of a bat. Basically if the bat is broken in too much it could go under the allowable pressure and be deemed illegal but I have rarely seen a bat rolled go under the minimum psi number. Some keen umpires, coaches, or league directors could possibly tell a bat was rolled by a visual check. They could see thing such as roller marks on a bat. These would run along the bat parallel or perpendicular where the roller has “cut” into the top layer of composite. Most experienced bat rollers will not leave these marks.
Bat rolling is the accelerated break in of a composite baseball or softball bat for the purpose of better performance. It comes down to a few weeks of batting practice or a few minutes in a bat roller to achieve the same goal.