I remember seeing this concept many moons ago as a web page, but it has since been refined into what is probably the most demanding (literally) writing application out there.
The app launches with a full-screen editor and a countdown timer. You select the amount of time you want to write and start typing away. Sounds like a pleasant experience until you stop writing to look at your tweets and gasp in horror as your wonderful prose is deleted by the app.
It'll delete your writing if you exit the app too. No messing around here. Write or die. It's around $2 on the iOS app store.
It's a little like chaining your muse to the back of your open skull while climbing the caldera of an active volcano.
Check it here, if you dare: http://www.tuaw.com/2013/02/19/daily-ipad-app-flowstate-is-a-brutal-writing-app-that-produces/