Rules Commander
Upkeep Triggers and Extra Turns
The upkeep step does nothing for most decks on most turns. A deck built around upkeep triggers is the exception, and it wants as many upkeep steps as it can get.
What the upkeep step actually is
Upkeep is the second step of a turn, after the untap step and before the draw step. State-based actions are checked, and then any ability that says "at the beginning of your upkeep" (or, for a symmetrical effect, "at the beginning of each player's upkeep") is put on the stack in whatever order the active player chooses. Everybody gets priority to respond before the game moves on to drawing a card.
Most turns, that step is a formality: a Phyrexian mana upkeep cost to pay, maybe an echo trigger, and that's it. A strategy can be built on it because the step itself can be repeated, and every upkeep trigger already in play fires again each time it is.
Multiplying the step, not the trigger
An extra upkeep gives you a second copy of every upkeep trigger already on the board. That's the mechanism Obeka, Splitter of Seconds exploits. Whenever she deals combat damage to a player, you get that many additional upkeep steps after combat, and every "at the beginning of your upkeep" ability triggers in each of them exactly as it would on an ordinary turn. A deck with zero upkeep triggers gets nothing from the extra steps. A deck with five gets five more triggers per extra step, and one hit at her printed 2 power is two extra steps.
That's why a card like this is a multiplier rather than a value engine on its own. The payoff scales with how many upkeep triggers the rest of the deck is already running, which is why you build the deck around the strategy rather than splashing the card into an unrelated list.
How extra turns fit in
A Time Warp effect grants a whole extra turn, which comes with its own untap, upkeep, draw and combat, so it also re-triggers every upkeep ability on top of everything else an extra turn gives you. Extra turns and upkeep triggers are still different strategies that support each other: an extra-turns deck wants more turns for the mana, the cards and the combat step, and an upkeep-triggers deck specifically wants more of the one step, which an extra turn is an expensive way to get and a card like Obeka is a cheap one.
Cards that turn the upkeep step into a plan
Phyrexian Arena ↗
At the beginning of your upkeep, you draw a card and lose 1 life. It's the plainest example of a trigger worth multiplying, because every extra upkeep is another card.
Obeka, Splitter of Seconds ↗
Whenever she deals combat damage to a player, you get that many additional upkeep steps after combat. That multiplies every upkeep trigger already on the board rather than adding a new one of its own.
Paradox Haze ↗
Enchant player. At the beginning of the enchanted player's first upkeep each turn, that player gets an additional upkeep step. On yourself, it fires every upkeep trigger you control twice a turn.
Frequently Asked Questions
What actually happens during the upkeep step?
State-based actions are checked, then triggered abilities that say "at the beginning of your upkeep" go on the stack in the active player's chosen order, then players get priority to respond before moving to the draw step. It's the first real decision point of a turn, and it's also the point most players skip past mentally because it usually does nothing.
Do extra upkeeps trigger "at the beginning of your upkeep" effects twice?
Yes. An effect that grants an additional upkeep step causes every ability that triggers at the beginning of an upkeep to trigger again during that extra step, because the game doesn't distinguish between a normal upkeep and a bonus one. That's the mechanism Obeka, Splitter of Seconds and Paradox Haze exploit.
Is an upkeep-triggers deck the same thing as an extra-turns deck?
They overlap but aren't identical. An extra-turns deck (Time Warp effects) is chasing more whole turns, and an upkeep-triggers deck is chasing more of one specific step. An extra turn does incidentally grant an extra upkeep along with everything else, so the two strategies support each other, but a deck can lean entirely on multiplying upkeeps (Obeka, Splitter of Seconds) without ever taking an extra turn.
What's the best commander for an upkeep-triggers deck?
Obeka, Splitter of Seconds is the sharpest build-around. Whenever she deals combat damage to a player, you get that many additional upkeep steps after combat, and every upkeep trigger you control fires again in each one. She does little by herself, though. She needs a deck already stacked with upkeep triggers to be worth casting.