Today (technically last night) I learned what happens when a freewheel fails (i.e. starts spinning freely in both directions), and as a result what you expect to be a mighty forward pedal stroke encounters no resistance. In a nutshell, you wind up off the pedals, running down the street while straddling the top tube, wondering exactly when you're going to fall and trying to ignore the fact that your somewhat sensitive male parts are repeatedly striking said top tube.

Fwiw, it's unpleasant.