erm.. is Nissin Di866 able to trigger wireless by a Nikon D40x?
yeups. but you have to set the nissin Di 866 to slave function first. then pop up your pop up flash and set the power to the lowest rating so that the pop up flash wun't be to harsh. and oh yeah the nissin flash has to be infront of your camera because it fires off via infra red if im not wrong.
hope this helps