The combo begins when you enter the end step with Dakmor Salvage in hand, along with at least 7 other cards, Gitrog on the field, and 2 mana open. The combo ends... eventually.
One will note that when a triggered ability happens in the cleanup step, players get priority, then when the step ends, another cleanup step begins (514.3a). This means your discard can include Dakmor Salvage, which triggers Gitrog's ability, causing you to dredge back Dakmor, and because a triggered ability happened, there will be another cleanup step, in which you can do the same thing again. If you milled a land while dredging Dakmor, you get to draw a card as well, passing to the next cleanup with nine cards in hand instead of eight.
This can continue indefinitely, until you hit Kozilek on the dredge, causing your graveyard to shuffle back into your library so you can keep going.
You eventually need to hit a land on the dredge, causing you to draw Shallow Grave (or Corpse Dance if you have 3 mana open) -- while this will happen on its own eventually (just due to probability), it will take a while, as you not only need to reach it before Kozilek on a pass through the deck, you need to DRAW it rather than mill it. You can help this out if you draw Noxious Revival, putting Shallow Grave from your graveyard to the top of your library right before a draw.
Once you have Shallow Grave (or Corpse Dance) in hand, you need to get Psychosis Crawler into your graveyard before Ulamog, have it be the top creature card in your graveyard, and have nine cards (rather than eight cards) in hand, including the Dakmor and the Shallow Grave. Cast Shallow Grave on the Crawler, then continue the Dakmor engine with the crawler out, causing your opponents to lose one life each time you draw from dredging out a land.
There's another combo here that's faster in execution, but less common to get the pieces into place - Skirge Familiar and Dakmor Salvage in the main phase can let me run through the combo in main for infinite black mana, and go until I can just cast Psychosis Crawler. Far less interesting, but an option.
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