CBJ explained it on his radio interview today. About the 4:35 mark. It was a punching drill and he blew his bicep. It sounds like the hands drill they run.
https://soundcloud.com/espnnashville/ut-head-coach-butch-jones-talks-with-darren-and-derrick
https://soundcloud.com/espnnashville/ut-head-coach-butch-jones-talks-with-darren-and-derrick