The Freddie King Chord trick is a variation of the tritone accompaniment we’ve seen earlier.

You play the tritone interval (major third and flat seventh of the chord) on the third and second string.

Going from the I chord to the IV chord, the note on the fourth string is the tonic of the I chord, going to the fifth of the IV chord.
Which is the same G-note.

For the V chord we only have to move the IV-chord structure two frets up.