To Arms!

Two effects bundled into one instant, and the trick is noticing they want the same battlefield: a board of creatures whose tapping pays for something. Untapping all your creatures reads like a defensive tool, and it can be one (refresh blockers you tapped earlier in combat, or untap a clutch of mana dorks to hold up a response), but the line does almost nothing in a vacuum. It needs creatures already tapped and worth untapping. The cantrip is what keeps the card from being dead when the untap does no useful work: spend the two mana, draw, and the floor is a cycling cost rather than a blank. The ceiling lives wherever your creatures carry repeatable tap-cost abilities, mana-producing dorks chief among them. Untapping a fistful of them at once and replacing the card you spent is a combo enabler dressed as a combat trick: trivial alone, decisive next to the right activated abilities. White rarely gets to draw on demand and rarely gets to generate a burst of creature activations in a single beat, and this does both at instant speed, on an opponent's turn if it wants to. That combination is precisely why it goes unremarked for years at a time, right up until someone untaps every creature that taps for mana and the game ends a turn early.