The red box is shown to be empty

The yellow box is shown to be empty

The yellow box is put inside of the red box

Out pops a lovely assistant

"Shhhhh.  Don't tell.  We've turned the illusion around to show you how it is done.  You'll be seeing the 'Backstage' view of how we do it.  I start by hiding behind this red box when they show that the yellow one is empty"

"Then I climb inside the yellow box while Allen blocks the view.  After I'm inside the yellow box the audience will see that the red box is empty."

"The yellow box (with me still inside of it) is now put inside the red box"

And out pops the lovely assistant... er... I mean Dick Clark!

How'd that happen?  Oh well.  That's the way it goes!