Apparently, their software will be analyzing the number, speed, and rhythm of keystrokes along with reliance on or dismissal of spell-check. ...and frequency of mistakes and back-spacing.

No surprise to any of us, I think, that when becoming depressed our speed slows down and long messages are too much work, so short, slowly-typed messages will indicate impending depression. Whereas, messages typed quickly with little regard for correct spelling (who can be bothered when you've got so much to say?) will signal (hypo)manic tendencies.

There's probably more to it than what I've summed up here, but that's the gist. I tried to download it, but I don't have my payment details set up in the app store, so I'm out of luck even though it's free.