Flex works by creating “patches” to apps. First off, it gets the headers from a library that you want (such as ChatKit or UIKit). It allows you to choose the return value of the functions. So, for example, if the return type is a “BOOL” then you can either choose FALSE or TRUE for it. It then uses MobileSubstrate to patch it in the running processes.
Flex is a powerful tweak that lets you create and share patches for iOS apps. Community Development Team member tw23 has taken the trouble of explaining what this tweak does and how to use it. Head over to the forums and give his guide a glance, you may find that this tweak may just be worth purchasing!