Example: when you do a search and click a airline link to purchase the ticket from the first URL contains a forward parameter:

https://www.kayak.com/book/flight?code=1945498033.FWACDsQKhR.0.F.ORBITZAIRSYN,ORBITZAIRSYN.73702.d454964dc0e43df142432c90a5fd16a4&sub=E-150470716a7&resultid=d454964dc0e43df142432c90a5fd16a4&sr=1&pg=0&cp=0-4-5210&sc=4128

and secondly once the forward is complete:

http://www.expedia.com/Flights-Search?trip=roundtrip&leg1=from:JFK,to:IST,departure:11%2F15%2F13TANYT&leg2=from:IST,to:JFK,departure:12%2F15%2F13TANYT&passengers=children:0,adults:1,seniors:0,infantinlap:Y&options=cabinclass:economy,nopenalty:N,sortby:price&mode=search&mdpcid=US.META.KAYAK.MULTIBOOK.FLIGHT&mdpdtl=FLT:JFK:IST

It seems that there is a 'kickback' model here once a ticket is purchased. Any ideas/views from a technical perspective would be appreciated.