The GIF in the README.md probably does a better job of explaining the core mechanic than words can. But basically: We mask out a sentence, leaving only as much detail as the user needs to actually complete it, and then have them fill it out, word by word.

I'm showing it less because Finnish itself is interesting to most people, but because I think the general mechanism here could be useful for many other foreign language learners out there. I would especially recommend watching the YouTube video linked on the page, as I recently added a real practice session mode as well as a timer I've found to be quite useful in practice.