The Ninth Doctor

An extra upkeep step, granted passively and keyed to your untap, is the payout here, and it is quietly one of the more valuable things a body can hand you turn after turn. Most cards that grant additional phases either charge real mana each cycle or ride a fragile creature that folds to the first removal spell; this one keys off your untap step, so once the engine is online it costs nothing to run. Read the condition it is built around: the ability fires when the creature "becomes untapped during your untap step," which means it has to have been tapped, has to survive to your next untap, and has to actually flip back upright to pay out. Hold it down under a Winter Orb effect or a stax lock, or simply never tap it, and it gives you nothing; the reward is reserved for a body that goes to work and comes back. A 2/4 with haste is not a clock, and it does not need to be, because the point is accumulation. Every upkeep trigger you own fires twice, every drain, draw, or counter-doubler keyed to your upkeep runs on overtime, and any suspend or age-counter engine advances a full step faster than the table expects. Doubling one of your phases for three mana and no recurring tax is a bargain slow-engine players tend to undervalue until they watch the loop suddenly run at double speed.