The way the pre-registrations work are dictated by the Registry, but they are generally done in two stages.
Sunrise and Landrush.
Sunrise are for trademark holders. Since there can be multiple trademarks for a given name (each in a different trademark product space - like Apple Records and Apple Computer), any conflicts are resolved in a round robin approach, where the different registrars submit one name at a time to the Registry and the first to submit a valid trademarked name from a valid trademark holder gets that domain.
Landrush is the same thing but for the general public.
If you have pre-registered for sex.us, and so have 100 other people, the first registrar that submits sex.us in the round robin process (in the case of .US on the 24th of April) gets it.
Most registrars will only accept one pre-registration request for a particular domain - however some will accept as many as are submitted. Most will also refund you completely if you don't get your domain, however some will charge a fee - whether you get your domain or not.
Does this help?
-t