I don't do it myself, but I believe yes it is as simple as backing all the horses you think could win it and therefore having several horses running for you (i.e. a book) rather than just one or two. With the prices being much longer now (for a race like the RSA anyway), you can have several covered and still make a profit if one of them wins. Getting the horses at bigger prices also gives you the opportunity to lay them at a later point if you want to as well - e.g. someone covered Bob Ollinger at approx 20/1 - he won't be that price if he wins a novice chase, likewise Gaillard Du Mesnil (40/1), Bravemansgame (20/1) and several others, so if you get them in early, you can still make a profit from backing so many as long as one wins!