Chord function describes how each chord in a key behaves. Chords that behave in a similar way are made family. The three chord families are based on the main function chords (I, IV and V).

The one chord (I) is home, and every chord has a relationship to the home chord. The four chord (IV) "feels" like we're moving away from home. The five chord (V) prepares for our return home. Chords that share the same notes are grouped together. Chords I, iii and vi are home. The ii chord is in the same family as the IV, and the vii chord is in the family of the V.

Each chord in a progression belongs to a family, and we can exchange chords for others in the same family without changing the feel. When you move from one chord to another in the same family, you are extending the feel from that family. In the video, the modified progression moves from the one chord to the three chord, then the six chord and back to the one chord in the beginning of the progression. This is the chord family technique where I extend the feeling of home and sense of stability by using chords from within the home base family.

When the four chord is exchanged for the two chord in the IV-V-I progression, you get the ii-V-I. In both progressions the chords move from family to family based on their functions. All the chords work together to create and resolve tension.

https://youtu.be/wXxBkEOc0eY