Slipknot concerts are gloriously grisly scenes of mutilated masks, vicious pits and hellish alt-metal. “Disasterpiece” and “Surfacing” aren't songs so much as industrialised rampages powered by strangled riffs and oil-drum percussion. Growling frontman Corey Taylor also proves a charismatic showman when he whips fans into a near-homicidal frenzy on the chugging rager “Spit It Out”.