Just try and play a regular G chord with your middle, ring and little finger while your index is barring the fifth fret. You should be playing this: 875558. That’s a C chord.

It’s not a very easy chord to grip, but that’s not the point. The point is the “geometric shape”.
So the “bottom part would be 87555X and the “top” part would be XX5558.

Hi Daniel, Just translating what Brian does in the video, so have attached the CAGED – G Shape visual. In the course, the CAGED G shape is being used for the B chord. Brian makes a bar chord across the 4th fret and places his pinky on the 7th fret, 1st string. When you play it, the strings that are unfretted don’t get played (xx4447). He also bars the 4th fret and places his ring finger on the 6th fret – 5th string. The 1st and 6th string are unfretted and don’t get played (x6444x). He also mentions the B Chord triad on the 4th fret playing only the 2nd, 3rd and 4th strings (xx444x). Jean-Michel has also given 2 other options of playing the G Chord shape and so by using 76444X and 764447, they can also be played as a B Chord. As stated, the point is knowing the geometric shape despite the difficulty in playing these chord options.