A master tactician, engineer, and fighter pilot who changed the way the USAF in particular understood air combat. His main work is called the 'Aerial Attack Study'. Like many truly influential fighter pilots, rather than rack up the kill count (the Korean War was hot), he spent his time truly understanding his trade, and imparting his knowledge upon others. He got his nickname from his ability, as an instructor, to start with a student on his 6, and, by 40 seconds after the "Fight's On!" call, he'd be on the student's 6. His studies into energy and aerodynamics eventually changed the way the USAF fought in the air.
A modern example of how he understood energy states and his own aircraft's strengths and weaknesses can be found in the DCS F-14 thread under the video "Tomcat: Dogfight Techniques" where someone today demonstrates his similar knowledge of the DCS F-14.