Knacks occur when someone has an intrinsic connection to a certain place. Over the course of many years in one place, the person develops an understanding of the location. Eventually this person can create one subtle effect (rarely more), but only when in that location. Knacks are very common in cultures that are non-nomadic. For example, a grandmother who can makes perfect pies in minutes, or someone who simply cannot get lost in a forest in which they grew up. Most people, often even the person with the knack, do not even consider these effects to be magical.
Trained tappers however, do recognize this as a very minor form of tapping, and thus call it “ancestral tapping” in deference to the supposed ancestral bond the individual carries with that one place.