The unconscious and unintentional learning of complex information is termed as implicit learning and its end result is tacit knowledge. We see that unlike explicit knowledge, it is difficult to express tacit knowledge verbally. It is said to be ingrained in our unconscious and is thus hidden from the conscious.
The concepts of implicit learning and tacit knowledge very well explain how experts acquire such superior skills and why is it difficult for them to transfer their expertise to a non-expert. Experts gain knowledge in their area of expertise after repeated encounters with the task. They have experienced the situation so many times and they know what course of action to take even if they cannot explain why they are doing what they are doing. It resides in their unconscious as tacit knowledge and they themselves are not aware of all the things they know. It is nearly impossible to extract learned patterns from them and such expertise can only come through a similar learning process.
One example we discussed was from the book Blink by Malcolm Gladwell about "The statue that didn't look right". It talks about how a lot of expert archaeologists could instantly tell that there was something wrong with the statue kept in Getty's museum but nobody knew what it was. They could never verbally tell what the fault was. It just didn't look right to them. Further investigations proved their claim and the statue was found fake. This example further strengthens the role of tacit knowledge in the performance of experts.