Source: www.reddit.com

Alice_D: "The app is called Flex, it is available only on jailbroken phones and allows to get the list of all classes and methods with their signatures from any app. It also allows to hook any method and modify return values/parameters, thus allowing to create tweaks. I mostly use it to get paid features for free though, you won't believe how many apps rely on simple IsPremiumVersion(bool) :P"