Download Play?

I was playing Sonic Rush via download play with my brother and had a cool (and probably not possible) idea.

How does download play work? Is it possible for homebrew and maybe even emulators to use this feature?

It'd be sweet to play a game like TMNT or Double Dragon via download play with a friend and seeing on how some DS *unspeakables* can download play with un-flashed DS's I thought that perhaps homebrew wouldn't require a flashed DS either.

i dont think you can do download play
im not sure though, try googling it and see what you find
i know some use wi fi so maybe it does work