Developed by Steve and Jay McDonald (Cousins), Fjax is the lightweight, cross-browser, rapid-development methodology for Ajax-style web development which utilises the Flash engine to handle the updating of XML/HTML content in realtime.

Fjax is based around using Flash as an enabling technology, it is not about "Flash movies" or presenting your content in Flash, or having Flash as part of the presentation layer at all. (Fjax website)

Fjax uses Flash to handle XML simply because it is so good at it! Flash is quick, small, cross-browser compatible, and it has a huge number of users.

For more information on Fjax and its development visit the Fjax website. The Fjax website gives a good over view of what Fjax actually is and contains some clear examples of what it can do (Fjax Examples).