Diplomacy rule application
Here is an example.

A BEL-HOL
F NTH C BEL-HOL
F HOL-BEL

Two pieces can trade places if and only if one of them is convoyed. So in the above examples, BEL and HOL trade places because BEL is convoyed. Without the fleet to convoy, the two pieces in HOL and BEL would bounce and stay in their respective palces.