Contango uses the concept of flash swaps to make the protocol capital efficient. Let's say a trader wants to buy 1 ETH with 10000 DAI. Without using flash swaps the trader would need to first give DAI before receiving ETH. With flash swaps the trader could get 1 ETH first as long as the 10000 DAI are given back in the same block. If that's not the case the transaction is reversed. This allows the trader, or a protocol such as Contango, to perform some actions between receiving the 1 ETH, at the start of the transaction, and giving 10000 DAI at the end of the transaction.