If this is the same concept as what I refer to as tromboning. Hope I can explain this clearly enough.
Lets say you connect two PBXs or switches via Tie trunk, a WAN or LAN to connect two sites for example. A call comes in on one site and is transfered to the second over the tie, 1 trunk channel is in use. When the call is tranfered back to the first switch a second channel is in use and the loop or hairpin is created. Some switches have anti-tromboning so both channels are then cleared