I believe it was developed for the US Department of State to crash train people who were going to work overseas. It isn't silly vocab drills. It has you reading entire literary passages and holding minutes-long conversations also immediately. None of the "get it right before you move on" stuff, either. You're absolutely going to fail, just like if you were really in the other country trying to talk and listen and not getting anything, but keep at it, and you will eventually get it. You can't get true immersion from an app, but this is as close as you'll get.
It's not freemium, though. You have to pay.