Wireless Distribution System. Basically both access points work collectively to create one single wifi network.
http://en.wikipedia.org/wiki/Wireless_distribution_system
It actually works fine with a powerline internet connection from the upstairs router to the downstairs router (used purely as my 2nd WiFi access point) so not sure I want to change it. Both devices are right at the edge of my connection points, so joining the 2 devices together via WiFi I suspect would be problematic as it will need a solid WiFi connection between the 2 devices, which is the problem really as my lounge just doesn't get the signal from upstairs due to a 2 foot thick wall stopping the signal getting into the lounge.
Or have I missed the point of this (highly likely as you might be working out
