Question: How do business rules relate to knowledge retention?

The classic test of whether knowledge is tacit or explicit is this: If you lose the person, do you lose the knowledge? Clearly, you want basic business know-how to be explicit, so a basic principle of the Manifesto is ...

3.3. Rules must be explicit. No rule is ever assumed about any concept or fact.

Rules capture and encode operational business know-how in a form that can be retained, managed and re-used.

What are rules really about? A well-expressed rule is based on terms and facts (or more accurately, noun concepts and verb concepts). These concepts represent the basic stuff of the business - operational-level things that are talked about, managed and processed day-in and day-out, often many, many thousands of times. Rules provide criteria that guide this operational activity in a consistent way. So the Manifesto emphasizes ...

3.4. Rules are basic to what the business knows about itself - that is, to basic business knowledge.

In business, of course, knowledge is not an end in and of itself. Rather, the goal is consistent application of the knowledge - as well as its continuous improvement. Achieving these goals requires that the people who understand the knowledge - business people, subject matter experts, and business analysts - be able to work with it directly and effectively. After all, the true test of knowledge quality is not whether an application program runs, but whether you get the right (or best) results. So the Manifesto states ...

9.3. Business people should have tools available to help them verify business rules against each other for consistency.

In the plainest possible terms, IT professionals simply shouldn't in the business of determining whether business logic 'works' for the business.

References:

[1] The Manifesto is free, only 2 pages long, translated into 15 languages. Have a quick look (or re-look!). No sign up required. Well worth your time.