[xmonad] action of "doFullFloat" upon pressing a key

Dear xmonad users, I wasted several hours trying to find this out myself -- any help is highly appreciated. My xmonad.hs has myManageHook = composeOne [ transience , isFullscreen -?> doFullFloat ] This makes $ xpdf -fullscreen something.pdf display on the whole physical screen. However, it doesn't seem to work with video players like vlc and xine. I would like to have the effect of doFullFloat upon pressing a key. How would one tell xmonad to do that? thank you, Christoph