[skybrian]

It's built on large files that act as bingo cards. You have a limited time to respond when you get a bingo. The more cards you have, the more likely you are to win.

They are designed so that precomputing the bingo card is expensive but once you have them, checking to see if you have a bingo is cheap. It's thought that nobody will be able to compute a winning bingo cards on the fly ("grinding"), so it will be cheaper to store and reuse them once you have them.

(I don't know the details of the algorithms though.)