Pardon my ignorance but what is flex?
think of holding a golf driver and wiggling it and you will see the shaft "flex". While considerably less marked, the handle on a bat can flex.
The reason that, for example, the carbo handles are popular is because they are very stiff. The stiffness in the handle causes less energy loss through the shot and thus, in theory better performance.
This is different to a golf club where the flex is used to increase club head speed in slower swings.
The downside of a stiff handle is that it mishits cause more vibrations.