In-app Debugging and Exploration 

FLEX is a unique debugging tool to view and modify a running app:

“FLEX (Flipboard Explorer) is a set of in-app debugging and exploration tools for iOS development. When presented, FLEX shows a toolbar that lives in a window above your application. From this toolbar, you can view and modify nearly every piece of state in your running application.”

This is also a good way to get your head around someone else’s code base.