Crawling zombies are zombies without legs that crawl across the floor, the way I use it is to find a space and walk in circles, that way the zombies will be following me and accumulating behind me, when I notice that the zombies have stopped if it accumulates behind me, I throw a grenade in the middle of this crowd, so some zombies will survive but without legs and just throw another grenade to kill the 5 or more crawling zombies.
note: I recommend doing it in higher rounds like 7 or 16 where there are more zombies so that the chances of crawling zombies are greater.