Watch the Scale Pattern p-i-m-i online guitar lesson by Adrian Legg from Fingerstyle Revisionist: Origins

Simple enough musically, notice how the middle (M) note stays put and is gradually eased under by the rising thumb (P) and index (I) notes. In a lot of fingerstyle playing, the notes are where they can be hit conveniently rather than where they might fall logically in linear thinking—i.e., the way we might have learned with a pick.