There are two main reason for that choice:
1. it was used by Michelino Mavatiku Visi in the Kaful Mayay solo that I already posted, so this explains how to play lines like that one
2. it is one of the most basic and powerful things one can learn on any instrument.
To make things manageable I limited the choice to just one scale, C major, and to simple 1-3-5 chords (no 7th or other intervals).
You can find the explanation of the harmonization of the major scale anywhere, on the internet (search "harmonization major scale" or "harmonisation gamme majeure") or in any book about music harmony.
The harmonization results in this chord sequence: C, Dm, Em, F, G, Am, B dim.
So all we use here are simple major and minor chords (ok and one diminished).
The first sequence consist of playing the C major scale using a chord for each note of the scale. The first set of chord shapes starts from the C major in the first position, with the root on the 5th and 2nd string:
This is the chord shape used for all major chords in the sequence: C, F and G
The next note is D, and it is a minor chord, for which I use the following shape:
This is the chord shape for D, E and A in the sequence.
The B dim is this:
We don't really need these big 5 string chords, I only use them to start from something familiar (the first C chord above should be quite familiar).
Next step in the video is to play only sets of three strings out of those chords. If you notice, using those chords every three adjacent strings have all the notes in the chords, i.e. they are basic triads or inversions of basic triads.
So we can use for our solos the chord sequence we just saw, but playing only the 2nd, 3rd and 4th string, and that's what I try to do in the video: playing the harmonized scale using only the 2nd, 3rd and 4th string with the following shapes
this one for major chords C, F and G (in red is the root)
this for minor chords D, E and A
and this for dim
And again the sequence is: C, Dm, Em, F, G, Am, Bdim.
Then I do the same with the first three strings, and the triads are these:
So, at this point we have already two possible voicings (harmonization) for any line you want to play in C.
We can get easily to six.
Using the following shapes for major, minor and diminished chords with the root on the 5th and 3rd string
again very familiar chord shapes (at least the major and minor ones), we can get two more sequences of triads by playing only the 2nd, 3rd and 4th strings
And similarly, the last two sets derives from these other familiar chords, root on the 1st, 4th and 6th string
And here the last two sets of triads played in the video, sub-sets of the previous chords
In the video I just try to play the harmonized scale up and down and to highlight the root, but of course that's just for the demo, it's not exactly the most musical use of this concept.
You can simply harmonize a melody (try a simple children song, or even better a classic congolese rumba from the fifties), or you can play rhythmic lines by breaking down the triads the way Lokassa Ya Mbongo does (although he usually uses wider chord positions), or you can build single note lines moving those shapes around. If then you start thinking that if you play the triad one third down the scale you are eliminating the root and adding the 7th a lot of possibilities open up, try playing every other triad instead of the straight scale.
Whatever use you find for this it is certain that, talking about congolese guitar, it was used a lot in the slow melodic rumba of the origins and that Michelino Mavatiku Visi knows this stuff very well.
As always, if you have any comment just use the contact form and I will read it and answer as I can.