The information on the credits page is all that is available, it should be enough to understand the use of credits.
You gain credits by posting threads, or replying to threads. As of now, you can only spend credits on unlocking downloads. There are a few up-coming games that will allow you to play with your credits.
To gain credits faster, you can post a few leak threads. Which is where you gain premium content (Paid plugins, paid themes, paid software) and then post it on here for people to unlock and download. You get 8 credits for every unlock.
You could also post a tutorial for something and lock it so people have to unlock, but the tutorial has be to pretty high quality for it to be worth 8 credits.