Here’s the gist of it: a game will go on sale at a deep discount, but only a very limited number of copies will be available to buy. At the moment, Ultimate General: Gettysburg has a -75% discount, but there’s only 40 of them available.
In the time between starting and ending the writing of that sentence, the game above went on sale and was sold out. This is especially impressive when you consider that the big spenders in America aren’t awake yet, so the deals will go faster as the day goes on.
To top all of this off, buying these deals will get you free games. At the moment, buying any 10 of the available games will get you a free title. What that game is, and what the next titles that will go on sale, we can’t say.
What we can say, however, is that the free games have the stipulation that “the ones you don’t own are prioritised”. While that is a good stipulation, as far as those go, it does imply that you could wind up with duplicates.
In addition, each “tier” you get past will make the number of titles you need to buy lower. As explained by GOG:
For example: after these 10 initial purchases and your first freebie, you will need to buy 9 more games to unlock your second free game, 8 for the third, 7 for the fourth and so on until you receive one free game for every 5 purchases. When you purchase a bundle with multiple games, each one will count towards your next free game. Keep in mind that the stock of free games is limited. However unlikely, it’s possible they may run out. Additionally, only games that are part of the Insomnia Sale lineup count towards your total.
Looking at the FAQ of the sale and things get even stickier: the free game codes are only available until April 28th, 2016, meaning you’ll need to redeem them before that date. The sale will end when “stocks run out”.
As time goes on, the sale is getting more and more frantic, so keep GOG.com open in a separate tab throughout the day; we suspect the games on sale won’t be for long.