Originally Posted by Master of FiFi

There is also a situation with seniority trip trading. Say you try to swap to an open trip and a pilot junior to you also wants it. If a pilot senior to you wants the trip the pilot junior to you is trying to drop, it will give him the junior pilot's trip and give the pilot junior to you the open time trip to repair his schedule.

PDRed and legal, and it said that a lot of trip trading happens this way.