A simple way to explain this, all chords 4 steps apart are related to each other with interchangeable notes. eg. Em and G (as quoted in the vid) Bm and D , F# and Ebm , F and Dm etc . Count upwards if you are playing a minor chord and downwards if you are playing a major chord. This is an easy way to find the right notes for improvising.
btw
The opening guitar riff of "Johnny B. Goode" is a copy of the solo on Louis Jordan's "Ain't That Just Like a Woman" released in 1946
stolen