Nothing, internally it uses [hWaitForInput](http://hackage.haskell.org/package/base-4.6.0.1/docs/System-IO.html#v:hWaitForInput) which just returns false if there wasn't any input until the timeout. I use this because `cabal repl` takes a little bit longer to start up and I want to clear it's output.

It might be smarter to use a `threadDelay` to wait only once and then clear, though.