Flowstate allows you to choose how long you want to write for (5, 15, 30 min), then you enter a flow session. If you exit early, or stop typing for longer than five seconds, all progress is lost. The only way to save your work is by writing until the timer ends.
Compatibility: macOS 10.41 or later 64-bit
Mac App Store: https://geo.itunes.apple.com/app/id1051600144