A$AP Rocky grabs at his left leg and gasps with pain. “I did a front flip into the crowd last night—and landed on this kid’s head,” he explains, wincing as he limps across the floorboards of Jimi Hendrix’s old studio.

It’s 6:30 p.m. on a January Saturday, and Rocky’s on the top floor of Electric Lady Studios, the Greenwich Village recording complex Hendrix built just before his death in 1970. The front flip in question happened at Brooklyn’s Barclays Center, during a mammoth concert-qua-party, known as Yams Day, which Rocky...