Before she debuted her new song before millions on “American Idol’s” season finale, Beyoncé killed in rehearsals. In this candid iPhone footage shot by Jay-Z in his wife’s dressing room, B and her band run through her performance of “1+1” as her mother Tina Knowles and nephew Julez look on. If you had any doubt about the power of her live shows, this should make you a believer.