Here's How
The easiest way to explain it is this:
Go up to a department store glass door.
When you open it, stand so that you can see your reflection on the outside of the door.
When you get it half way open, look through the glass at what's in the store.
You will also see your own reflection in the glass, as well as what's in the store. Your reflection will look ghostly because you can see through it to the scene inside the store.
Imagine if you could push the door open so that it opens into the store, while you continue to stand outside.
Someone standing inside the store would see:
1. your reflection in the glass, and
2. what is on the other side of the glass,
But they WOULD NOT be seeing you directly, just your reflection.
This is how the ghost trick works in the mansion. You are seeing the reflection of figures that are above and below the ride path the doom buggies take.
What you don't notice is the glass that is between you and the real room you are looking down on. This reflects the images of the figures on the glass for you to see.
The tricky bit is that Disney has setup the figures to match the furniture in the room you are looking down on through the glass.
The figures are above and below you, litup with black light and other lighting effects, with a black backround behind them. This lets you just see the "ghosts" in the reflection, and helps them match up with the scene that is set in the ballroom.
I hope this helps, there are some behind the scenes pics at
www.doombuggies.com