A false flat is an optical illusion when your eyes tell you that you're on level ground (or even going slightly downhill), but your legs protest that this isn't the case. Of course, in of itself, it could just mean that you're tired. But if you turn around and suddenly find yourself flying along, and it's not because of strong winds, then it's a false flat. (You don't have to actually turn around to make it a false flat, of course.)