Complex sentence: Rock musicians refer to them as "power chords" and often include octave doubling (i.e., their bass note is doubled one octave higher, e.g. F3-C4-F4).
Split into simpler sentences: Rock musicians refer to them as "power chords". Power chords often include octave doubling (i.e., their bass note is doubled one octave higher, e.g. F3-C4-F4).