With its offbeat tempos and prevalent mallet-wielding, Jaga Jazzist may be the second coming of Tortoise, or of Shadowfax, depending on your temperament. Neither of those acts, though, ever rocked as hard as Jaga does on “Oslo Skyline,” a soaring track off the group’s recent What We Must, now available as a free download from its label’s website, ninjatune.com. This may be the most hard-core prog-rock thing Ninja has ever unleashed on its devoted, and generally downtempo-fixated, audience. (How proggy? Picture Godspeed You Black Emperor’s tour bus hijacked by Keith Emerson.) Jaga is a 10-piece Norwegian ensemble that mixes standard equipment (bass, drums, guitars) with old-school (trumpet, vibes, flute) and new-school (drum machines, wind controller, various keyboards) touches. More info at jagajazzist.com.