The signs seem to facilitate recall, helping you get past the point from where you know something to where you can pull it out of your brain.
The system works for a lot of reasons, though. One, it focuses on a very small vocabulary to gain a wide range of grammar, which is how infants learn. Also, it focuses on the core grammar, which again, infants do, whereas textbooks sweep huge areas of language (think articles) into a section of a chapter and either the learner gets it then and there or never.
Another key item is that it's immersion. Grammar isn't taught explicitly. Instead, simple situations are set up so the learner gets the satori and wants to learn more.
Check out the core video at whereareyourkeys.org
. There's a wiki. Also, languagehunters.org