A change of feel here. This is a very common rhythm in Cash’s playing, and he would often switch to this type of feel when he wanted to ‘drive’ the music more. Yes it’s simple, but oh so effective!
Muted strings play a big part in Cash’s style and they serve a definite purpose – early on in his career he decided he liked the sound of the snare drum and used muted strings as a way of mimicking it.
simple bass notes and bass runs are often used to connect one chord to another (the intro to Folsom Prison Blues is a classic example) and while the Telecaster-toting luther Perkins often took care of these details, you will hear Cash play simple bass runs as well.