Hancock was world-famous particularly for his great improvisations; he is a figure who will likely still be emulated in a hundred years alongside Pierre Cochereau, Olivier Messiaen and others. He wrote a book on improvisation which I confess I have yet to look at. Perhaps his most notable saying is that "salvation is a half step away." In improv, it's easy to get into a rut, and the way out of this is to modulate. More than once my somniferous doodlings have been rehabilitated by remembering this advice.
I never met Hancock and only heard him live once many years ago in an Anglican Evensong given by the St. Thomas Choir. It was one of those unforgettable performances, where the music is so beautiful and makes you feel so ecstatic that you feel like you've left your body. Time stops. Now time has stopped for Dr. Hancock, and we hope he will meet a Great Reward for a life well-lived.