Unintended Intro - iii-VI-ii-V Descending Four-Note Sequence
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It's basically a four bar sequence which repeats with rhythmic, melodic and directional variations the second time around.
The idea here is to maintain the integrity of the four note cell per measure / chord, while connecting the following measure / chord via a descending (mostly) whole or half step.
This results in the overall descending shape of the line.