D2W is a simple system that uses true/false logic to evaluate rules against the existing data and the continually updated D2WContext. The goal of D2W is not to provide sophisticated reasoning ability, but to provide the ability to describe in a declarative manner defaults for components, sizes, layout, etc., and to do so fast enough to be usable in a production system. In practice, D2W makes publishing a database on the web a trivial exercise by creating pages to query, list, display and edit objects. It does this by introspecting your data model and its context at runtime to tailor the display to the current condition.
D2W is not easily explained to those who have not used the technology. Part of the problem is that the target audience has changed over time. Initially it was designed to get beginning developers through the steep learning curve associated with WebObjects. The power lay in freezing auto-generated components and then modifying the resulting pages. This idea became closely tied to D2W, but it is actually for beginners and offers very limited (although interesting) capability compared to the potential. Anjo refers to using D2W at this level as "using the clicky thing", by which he refers to the WebAssistant, to make simple changes to your application's behaviour such as hiding, showing or reordering the display of your entities' attributes, or making changes to the page loaded for a given task/entity combination. So long as your application's requirements are simple, this may be as far as you need or want to take the D2W concept.
If you stop there, though, you are missing out on the vast potential that became apparent to the NeXT developers after working with the concept for awhile: the power of the D2W system lay in moving more of the information into the rules and away from frozen components and code. Unfortunately, developers who try to take an application through the transition from the WebAssistant modified behaviour to actually writing the rules that underly the assistant will find that the rules will no longer be "human-readable" because of the way that the assistant creates the rules. If you look at the HTML output of any common word processor compared to a hand-coded page, you'll have an idea what is in store.
What this really means is that there is yet another leap in the already steep learning curve of WebObjects to take D2W to its most advanced level. You should know going in that "in the real world you will need to invest a lot of time to get things working the way you want and maintain them until you understand what is happening in your application." The object of this document is to reduce the time you need to invest to understand the technology and the capabilities that it offers. If you decide to take this leap, your productivity gain by using D2W will more than offset any time investment you make to learn it. The first reason is that the D2W rule system allows you to create default rules and then specialize them. The second reason is that your application programming will begin to leverage Project Wonder which was created to expose the thinking that allows you to best exploit the power of D2W. If you really intend to learn D2W advanced features, you will be obligated to link in the Wonder frameworks to take advantage of its bug fixes to the stock WO frameworks and the vast number of additional components that you will get to use for free.