what to expect at a fraternity pref night

[Gusteau]

Actually, a decent number of schools have a more formal night at the end of the Rush Period, before bid day, that is sometimes called Preference Night. Typically you get dressed up, have dinner, etc. Depending on the structure of rush, going to this one house either shows your preference for that chapter over the others you've been rushing, or in more formal structured recruitment setting (where invitations may be handed out throughout the week) it is simply the last night of Rush before Bid Day. I don't know if that is what the OP is talking about, but a fraternity "pref" isn't really that unusual.

Regarding being weeded out after receiving a bid, in most cases you can be.