In addition to David's Hancock quote, there's this little gem of wisdom from John McLaughlin:
"When I improvise, I need to know everything and nothing at the same time".
He further explained that he practices to be able to have the skills necessary to achieve the ability to forget about technique, theory, etc. and just play.
IMO- that is the key to allowing yourself to be 100% in the moment and experience true freedom of expression when you play.