This is a simple Cash-style pattern that he would often use: root of the chord (A), strum, 5th of the chord (E), strum. There are many variations on this approach but this is the classic Cash move.
t2he5D27 chord – the root (D)Sistualaterrtn'satAedcowuitshtic the 5th (A) for the bass accompaniment.
The bass accompaniment pattern doesn’t always have to include open strings. On an E chord you have root (E) followed by the 5th (B), which appears on the 2nd fret of the fifth string. Plus, the pattern doesn’t always have to go root-5th: there are many occasions where you’ll hear root-3rd used as the bass pSaTttYerLnE,especially over G and C chords where it falls under the fingers.