The mat space at Gracie Humaitá Cedar Park was filled with the sounds of electric guitar, crashing cymbals, and the heavy panting of thirty jiu-jitsu students grappling. A timer ticked away the seconds of controlled violence under bright fluorescent lights. I couldn’t hear or see any of it, though. I was too busy fighting Tim Kennedy. Kennedy began our “roll” seated on the mat, all but inviting me to engage him. I stepped forward, knees slightly bent, and took grips on the collar and pants of his cotton gi, looking to advance my position and work past his legs. Earlier in the class, Kennedy had shown students how to initiate a takedown from a seated position. “I love starting on bottom,” he’d told the room, as if…