Grooving is a spiritual thing for James Gadson.1
On this track, the band grooved for about ten minutes straight. After that, Bill Withers put down his vocal part and the engineer manufactured a breakdown by soloing vocals and drums for certain sections.
If you listen closely, you can hear the bass bleeding into the drum mics during the breakdowns.
I dig this work-flow. When the album version starts, the musicians have already been deep in the groove for a few minutes.