The exercise opens with movement between two dominant voicings. It begins on an A dominant sound and the moves to the V of A with an E dominant sound. This quick change to the V of the original chord is very useful especially on long static parts of a chord progression. Both of the voicings use #9 chords because they give a bluesy feel to your rhythm playing. Dominant #9 chords, like the ones used in the open bar, use both the major 3rd and minor 3rd (#9) together. You can hear almost every blues soloist play around with these two different notes throughout their solo and it is one of the major characteristics of blues playing which is why they also work so well in your rhythm playing. The phrase ends with a D9 chord voicing (the IV chord of a blues). This chord is proceeded by another dominant chord that is a half step up. This functions the same way as the change from A-E in the first half of the bar.
The second phrase introduces superimposing a minor chord on a dominant sound. Using A-7 over the D7 can create a more open, modern feeling. This ii/V superimposition is very common especially when playing with a full band because the bass player will also be covering the original root. This idea ends with a chromatic walk up from G13 back to A.
The third phrase adds a bit more melody to your rhythm playing. This can be especially useful when playing rhythm behind singers to either accent the melody of fill in between the phrases. The most simple way to add melody is shown in the exercise using common shell type voicings with single notes played above. You can also try harmonizing a melody using voicings along the upper strings.
The lesson ends with a walking bass example. Adding in bass lines to your rhythm comping in duo situations is very common. This phrase uses shell voicings and chromatic approaches to create the bass line sound. You can also just leave out the chord voicings and play a bass line by itself. This is used frequently on more up tempo songs.