"Harmonic Rhythm -" What is it? It's the rate at which the chords in a song move through time.
Here is a song by Bacharach & David called, " Walk On By." This song is 20 (twenty) bars long. It's form is ABAB. Imagine a song twenty bars long, and, yet, it was a major hit for them. One never knows!
Here is how the chords move through time. A forward slash / = one beat. Also - = bar line. This song is in 4/4 time.
am7 / D9 / - am7 / D9 / - am7 / D9 / - am7 / D9 - so for four bars there are the same chords on the first and third beats.
gm7 /// - am7 /// - gm7 /// - am7 - so for four bars there are the same two chords on the first beat of each measure
dm7/// - am7 /// - Bbma7 /// - C9sus /// so on these four bars there are four different chords changes on every bar on the first beat - variety is extremely important!
Fma7 /// - Bbma7 /// - Fma7 /// - Bbma7 ///
Fma7 /// - Bbma7 /// - Fma7 /// - Bbma7 /// And the last eight bars is a repeated chord pattern on the first beat of the bar.
It's amazing what one can do with so little in terms of Harmonic Rhythm.