When they do a pre-parade, it's just a small snippet from a soon to be (or newly) released movie, that comes down the parade path a few minutes before the actual parade does.
I haven't seen one at DCA yet, but we were there for Enchanted's pre-parade, which involved Giselle in her (never used, pointless that she was in it, LOL) wedding dress in a white carriage pulled by a horse (with a poop-scooper CM walking behind with a decorated poop cart), just going along the parade route.
I saw pictures of the Bolt pre-parade with Bolt, the cat, and Rhino (in his ball I think) on a float going down the parade route.
It's basically just advertising!