If the two trades have the same terms, the payments and receipts of the first trade are offset by those of the opposite trade. The party on the other side of the opposite trade then holds the variability of payments and receipts that remains.
Consider a hypothetical example in which a sales entity enters into an interest rate swap with a notional amount of 10 billion yen and a fixed rate of 1% with a client company. The sales entity enters into an opposite swap on the same terms with a booking entity. If the floating rate is 1.5%, the booking entity receives 100 million yen fixed and pays 150 million yen floating, a loss of 50 million yen. If it is 0.5%, the booking entity gains 50 million yen. In both cases the sales entity's payments and receipts net to 0 yen.
"Booking" in "booking entity" means which entity's books a trade is recorded in. It is different from entering a trade's execution details. An opposite trade with a party outside the group is also called a back-to-back trade, but an intragroup trade adds regulations on its terms and specific treatment of counterparty risk within the group.