It's not a secret at all, and numgun didn't invent it, although he is probably the only one who uses it in mapping nowadays. The grenade-launching polygon technique's been used quite a few times on some of Chakapoko maker's maps as well as other mappers' (make a fat Only-Bullets-Collide polygon, stand in it, and throw a grenade. Tuck a scenery in to conceal the polygon so it looks professional, and you've got yourself an artillery cannon).
