I looked at the code for this once. It's sort of a hack. It generates a random puzzle, tries to solve it with a deterministic backtracking(IIRC) solver that detects situations where it's stuck. If it is stuck, it shifts some mines around in the place where the solver got stuck, according to some heuristics, and keeps solving. It keeps doing this until the entire puzzle has been solved, verifying that the puzzle can be solved without any guesswork.

I will second the recommendation. As someone who's wasted too many hours on minesweeper, it radically changes the game. Because I know there's a logical solution, I'm able to find patterns that I never found in the original, because I'd often assume it was just one of those guesswork situations. It's similar to how chess puzzles are easier than an equivalent position during a game. Because you know there's a neat solution, you're more willing to put in the legwork of searching for it. Learned a lot about the game simply by playing Simon's version.