Hello,

They are trips that people go to another airport, most likely in another country in order to take the actual journey that bring them to their final destination. People do this as they can travel in F/J at the same cost as Y if they do not go to that country to take the offer. They are commonly seen on A fares departing from an EU airport such as AMS, FRA, BRU, ARN, CPH... etc

For example I could have paid £3500 return to travel EDI-HKG in J nut I only have to pay £780 if travel from MUC in the recent promotion! All you have to do is to buy return ticket between EDI-MUC on easyjet or BA or even use your Avila points to redeem flight for positioning!

More information:
http://www.flyertalk.com/forum/13303030-post4.html