The main page (https://flawless.dev/) has a diagram/video that shows how it would work and it basically writes any 'side effects' to a log which is used to track their existence. If they exist in the log, you read them; if they don't you run the code that generates them and then log it.

It's interesting, but I can't imagine how it is going to work at scale, both in terms of managing state across a very large application and in terms of running thousands of instances concurrently.