Well, personally I've never had a DEILD, but I think this is how it works. You can only perform a DEILD after you wake up from a dream, that is why it is called a Dream Exit Induced Lucid Dream. You train yourself to not move after you wake up and keep your eyes closed and everything in the exact position. Supposedly if you can do that you will go back into another dream in a very short period of time. That is just what I think, so I'll be looking to see if someone has a more "official" answer.