At the beginning of each upkeep, each player exiles the top card of their library. The player who exiled the card with the highest converted mana cost takes an extra turn after this one. If two or more players' cards are tied for highest cost, the tied players repeat this process until the tie is broken.
12/1/2004 Each player must exile the top card of their library at the start of each upkeep step.
12/1/2004 Each time there’s a tie for the highest cost, only the players involved in the tie continue.
12/1/2004 If a player fails to exile a card, they can’t take the extra turn.
12/1/2004 If multiple extra turns are created in a game, the most recently created extra turn is taken first.
12/1/2004 Remember which player would have taken the next turn if Timesifter’s ability hadn’t triggered the first time. After Timesifter leaves the battlefield and all extra turns have been taken, that player takes the next turn.
12/1/2004 If two or more Timesifters are on the battlefield, keep a careful record of which players get which extra turns. With two Timesifters on the battlefield, two extra turns are created for each turn taken.
12/1/2004 A land card has a converted mana cost of 0.
5/1/2006 In Two-Headed Giant, triggers only once per upkeep, not once for each player.