Would a better variant be to kill the buffer if you actually moved focus from the text area? That way you can pause and think without losing flow, but there's still the threat that if you choose to distract yourself, there are consequences.
The 5 second thing can just halt the timer. So you have to actually spend 5 minutes writing, more or less.
An alternate version would only run the clock while keystrokes are being input. Sort of like watching a professional sports game. The time on the clock doesn't actually reflect the time spent on task (though the setup of this one does correlate, save for the 5 second timer leeway). It probably takes quite a bit more 'clock time' to actually walk away with five minutes of 'writing time' so to speak.
I think you're assuming someone who procrastinates by doing other things on their computer. People also procrastinate by getting up and doing something else. Part of the point is to keep you in your seat, rather than letting you, say, get a reference book to "just look up one detail" and end up reading it for hours.
The 5 second thing can just halt the timer. So you have to actually spend 5 minutes writing, more or less.