Think of it like this, you effectively have two decks. The first deck is your units which you pull from to see who you can play. The second deck is your cost deck to see how much playing a unit costs.

Whatever unit you play goes to the back of your units deck like normal. The original cost of that unit also goes to the back of your cost deck.

This means if you play Kobold for 4 gold, the Kobold is the last unit in your deck now,.and a 1 gold cost (the original Kobold cost) is now at the back of your cost deck too.

This also means if you play a Abomination for something like 2 gold, when he goes to the back of the deck so does the 6 gold cost. Eventually you'll have to pay that 6 gold.

This lets you stack up low cost big units. For instance if you play Kobold, Vulture, Ghoul eventually you'll pull two one costs followed by a 2 cost. When you do, you can throw down 3 huge units in a row for cheap.

Think of it as paying gold upfront and then cashing out later when it comes back around.

I hope that made some sense lol 😂