I believe the trick is to grind away enough of the head so that what remains is an "S". There is the spoke body, running into the bend, then the bend, then the mushroom is ground off all around except the side furthest from the threads. When done properly the last bit of the mushroom can be clearanced through the hole in the hub, but when the spoke is aligned to the nipple the head won't pull out.
Sounds as though you might have been browsing Ken Kiefer's site. If so perhaps there is a clear description there.