The following explanation is from http://www.pimsleurmethod.com/:
What is Principle of Anticipation?
"The Principle of Anticipation requires you to "anticipate" a correct answer.
Practically, what this means is that you must think about the situation and retrieve the answer from your own memory before it is confirmed in the lesson. It works as follows: The lesson will pose a challenge—perhaps by asking you, in the new language: "Are you going to the movies today?" There will be a pause, and, drawing on information given previously, you will say: "No, I went yesterday." The instructor will then confirm your answer: "No, I went yesterday."Before Dr. Pimsleur created his unique self–instructional teaching method, the attempt to teach spoken language was based instead on the principle of rote repetition, rote repetition, and then more rote repetition! Teachers drummed words into the students’ minds over and over, as if the mind were a record whose grooves wore deeper with repetition. However, neurophysiologists tell us that, on the contrary, simple and unchallenging repetition has a hypnotic, even dulling effect on the learning. Eventually, the words being repeated as rote practice will lose their meaning. Dr. Pimsleur discovered that learning only takes place when there is a meaningful "input/output" system of interaction between learners and native speakers of the language, in which students receive genuine information and then are asked to retrieve and use it in meaningful exchanges between individuals involved in real–life or simulated spoken communication. "
What is Graduated Interval Recall?
"Graduated Interval Recall is a complex name for a very simple theory about memory. No aspect of learning a foreign language is more important than memory, yet before Dr. Pimsleur’s work, no one had explored more effective ways for building language memory.
In his research, Dr. Pimsleur discovered how long students remembered new information and at what intervals they needed to be reminded of it. If reminded too soon or too late, they failed to retain the information. This discovery enabled him to create a schedule of exactly when and how the information should be reintroduced.
Suppose you learn a new word. You tell yourself to remember it, but after five minutes you can’t recall it. If you’d been reminded of it after five seconds, you probably would have remembered it for maybe a minute—then you would have needed another reminder. Each time you are reminded, you remember the word longer than you did the time before. The intervals between reminders become longer and longer, until you eventually remember the word without being reminded at all.
This program is designed to remind you of new information at the exact intervals where maximum retention takes place. Each time your memory begins to fade, you will be asked to recall the word. Through this powerful method, you progress from short–term to long–term memory without being aware of it, while avoiding the monotonous rote repetition used in traditional language courses.
So I will be on a new disc and all of a sudden information from a previous disc, say 6 discs ago, will come into play. For some reason I am able to remember it very easily. Everything seems natural. I can't recommend this program enough. They have it in every major language and you can probably find it at your local library. That's where I got mine. I checked them out, quickly burned them on my computer and made a playlist for all the discs. Then to make sure I don't use up too much memory on my ipod, I put an hour's worth of material directly onto my ipod and then rotate the tracks so that I have the most relevant lessons accessible for listening and then I just uncheck them off my library, and check the ones I will listen to next. So all together I'm listening to 40 minutes of brilliant language training everyday because of my commute!