The phases of each player's turn are reversed. (The phases are, in reverse order, ending, second main, combat, first main, and beginning.)
As long as there are more than two players in the game, the turn order is reversed.
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29 Feb 2020
Steps within each phase play out as normal. For example, although the beginning phase happens last in a turn, the untap, upkeep, and draw steps happen in their traditional order. (If it were the other way, combat would require psychics to get through.)
29 Feb 2020
Any even number of Topsy Turvys (Turvies?) will essentially cancel each other out and return the order of phases to their normal order.