Flow control in screen

A reminder for myself: Second time that my emacs session within screen suddenly didn't respond to Ctrl+s anymore. The command would somehow be piped through to bash directly and thus send a STOP signal. Unlocking the screen with Ctrl+q was easy but the new Ctrl-s meaning effectively killed my save-buffer command in emacs. While I noticed that I had hit some incorrect key combination I was not exactly certain which one. This happened before and I didn't have the time to check for the origin of the problem last time. This time I found the required hint. I must have hit Ctrl-a f accidentally thus activating flow control.