I explained it in detail in one of the Live Streamings where i shared Bengaluru meetup learnings. Assume you are long on a particular strike price and i am short. We decided to exchange our positions i.e. you would like to take short position and i would like to go long. In this case, say we exchanged 5000 contracts between us. In this scenario, Volume will be 5000 but change in open interest will be zero as there is NO new position is created.