At the end of this song, the music ducks out and Philip Bailey sings the "Keep Your Head To The Sky" line three times. The first time, he's backed by the full harmony, but by the end it's just him and Jessica Cleaves singing way up high in a whistle register. Bailey was a big fan of "Minnie Riperton," who could hit these high notes. He and Cleaves were seeing how high they could sing that line, and it ended up on the recording.
Cleaves was a member of Earth, Wind & Fire for a short time in the early '70s, appearing on their albums Last Days And Time
(1972) and Head To The Sky
(1973). She went on to work with George Clinton on his various projects.
As for "Minnie Riperton," two years later she had a #1 hit with "Lovin' You
," where she hit a big high note at the end.