The head = the recognizable part or that which identifies it as the song. It could still be there but you aren't listening for your favorite part of your favorite song. What you're listening for is the inspiration that happens in the moment, and that inspiration is not going to be the same from night to night. So if you bought a record and then went to the show, you should not be hoping to hear your favorite track on the album. You might hear an echo of it which helps draw you into the improvisation that follows, but if what you hear is exactly what was on the record it's not an improvised performance.

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Big PEMFin H & z's
"I ain't no entertainer, and ain't trying to be one. I am 1 thing, a musician." © Miles

"When the music stops he falls back in the abyss."