Typical life-cycle:
http://www.icann.org/registrars/gtld-lifecycle.htm
Most backorder services (especially the "big boys", Enom, Snapnames, Pool, and the like) have registrar "partners". In these relationships (check the backorder company's website), a domain that is already registered with a "prefferred partner" or "partner" registrar will not actually ever be deleted (not by the normal procedures anyway - and this has caused a huge debate amongst regular drop-catching-domainers over fairness), but just passed-over to that back-order company. In this case, the backorder company that is XXX-registrar's partner will have a very high (almost guaranteed) chance of ending with the name when it drops, assuming someone actually has a backorder from that backorder company for the name at the time of the drop.
Most of these same "big boys" (major drop service players) have an auction format, whereby every user that has a bid on the name by the time the name drops will go into an auction (most vary from 3-7 days) for the name. Only those members with a bid on the name by drop-time will be eligable to bid for the name. At the end of the auction time, whoever has the highest bid amongst that service's members wins the name and it registration for a year - registrar determined by the backorder service, for those that are not registrars themselves.
As to using multiple services - never hurts, however, as I have seen a few cases were a name wasn't snapped-up by one of the "partners"; and if the registrar it is at is not a partner with a drop service, then the name is just snatchd by the fastest computer going after it at the split second it drops:grinning-wink:
Advice: READ TOS carefully! Make sure what you are getting into with any of these services, and all fees/rules that may apply.
Good luck in the cut-throat business of grabbing falling gold the original domain owners didn't realize they had in thier hand but let expire:mischievous: Alot of times you get outbid by guys with deeper pockets.....Sometimes you can get lucky, when they don't want a half-good name...Sometimes, rarely but it does happen, you get real lucky and catch a diamond everyone else seemed too blind to see:grin: