In a recent New Scientist article, Harry Collins (author of "Tacit and Explicit knowledge) describes three types of Tacit Knowledge.
Tacit knowledge is "unspoken knowledge" and it remains unspoken for one of three reasons.
Collins describes Somatic Tacit Knowledge, which is the knowledge stored in the muscles, nerve pathways and synaptic connections. This is theoretically describable - "in principle, if not in practice, science could describe all of this. We still wouldn't be able to use it to guide our actions, because we aren't built for that". In other words, you can read a book that gives you the basic tango steps, but you can't learn tango from a book.
You can express the way to balance a bicycle as follows - "In order to compensate for a given angle of imbalance α we must take a curve on the side of the imbalance, of which the radius (r) should be proportionate to the square of the velocity (v) over the imbalance r~v2/α." - but the only practical way to learn this is to feel it (and to fall off a few times as well). The only way to transfer somatic tacit knowledge to someone is through long term coaching.
He describes Relational Tacit Knowledge, which is about social interaction and how this keeps some knowledge unspoken. Basically its the things you could explain but don't, for one reason or another. It includes secrets, the things you don't know that you know, and the things you can't explain because you don't know what the other party needs to know. This knowledge remains tacit for social reasons, and the work of the knowledge manager is to go through the social barriers and retrieve this knowledge through questioning processes - for example in Knowledge Interviews.
Finally, there is collective tacit knowledge. This is about the way WE work. Its about knowledge held socially and collectively. He gives the example of riding a bike. The mechanics of riding a bike are all about somatic tacit knowledge, but the knowledge of riding a bike in London traffic are collective and tacit; you need to understand the unspoken social conventions, otherwise the taxis and buses will get you. It is the collective tacit knowledge that the interviewer seeks for in team knowledge processes such as After Action Review and Retrospects, and the facilitator seeks to exchange in Peer Assists and Knowledge Exchange meetings.
Tacit knowledge is not always tacit because it CANNOT be made explicit, and the role of the knowledge manager is to access the knowledge that remains accessible.