Turnabout

The pivot point of half the broken mana engines of its era. The untap-all clause is what made it dangerous: point it at your own lands or mana rocks, untap everything tapped, and a wide board becomes a burst of mana big enough to end a turn. Pair it with permanents that generate more than it costs to tap them, and Turnabout stops being a tempo trick and starts being the loop that goes infinite. That it reads on any of three permanent types, and either taps or untaps, hides how single-minded its real use is. The "tap your opponent's stuff" mode is the fail-safe that justifies the instant speed; the untap-your-own-lands mode is what built decks around it. The design discipline is in the targeting: it untaps permanents of one chosen type for one chosen player, so it cannot both refuel your lands and freeze an opponent's blockers in the same cast, and untapping an opponent's lands hands the mana to them, not you. It does nothing without a board already worth untapping. A multiplier, not a generator: feed it a mana surplus and it doubles down, hand it an empty table and it spins its wheels.