He is doing artificial harmonics. He is stopping the string with his thumb, then lightly touching the string with his third finger. The most common ones I have come across are fourths and fifths. If you do a fifth (ex. + on G and 3 on D) then it will take the top note up one octave. But say you did a fourth (+ on A and 3 on D) then the bottom note will go up two octaves. There are other ones you will run across, but I don’t have experience with other intervals.
The concept is changing the length of the string. String length is measured from stopped point to stopped point (typically from the nut to the bridge). But, every time you put your finger down the string length changes because it is now your finger instead of the nut that would be the measuring point. This moves the location of all the harmonics.
Ex. If you put your first finger on the first A on the G string, theoretically (assuming you have a giant ass hand, or a third hand,… or I guess a capo) you will no longer have a middle G harmonic. You will gain a “middle” A harmonic that will be located over the A that is just above the normal middle G.
