*NSYNC electrified the world with their precision-grade dance moves and larger-than-life hooks, but digging into their catalogue reveals their willingness to play with pop’s boundaries. “Up Against the Wall” finds the boys flirting with 2-step; “I Thought She Knew” throws back to the doo-wop era with complex harmonies; and “Digital Get Down” pairs internet-curious lyrics with glitchy synths.