Passmember Preview is actually just a dry run in disguise. It's the day that they test all their new employees and rules and operating procedures, without having to deal with messing up a tourist's day at the park. So you can expect all rules such as no double rides, no loose articles on the platforms, etc, etc. to be in full and complete effect. Other than that they occasionally have special sales. These aren't anything huge. Also, at some point during the day they will run all three trains on any given coaster. They won't do it for long, because there normally isn't enough of a crowd for this to be necessary, but you can expect for it to happen for at least a little on each coaster. Just to familiarize the teams with the operation.
Hope that clears a few things up for you,
~Fur