You walk into a participating Lego shop, pick up a box of Lego, and walk over to the big screen. A video camera shows you your image. You hold out the box in front of you, horizontally, as if you're holding a tray.
The software sees the box, recognises which product it belongs to, and calculates the exact position of the box corners in three dimensions.
It then retrieves a 3D computer model of the assembled Lego model from its database, and projects a virtual reality image of the completed masterpiece onto the screen as if the completed Lego masterpiece is sitting on top of the box clutched in your little sticky hands.
You rotate the box, and on the screen, the 3D model rotates. Tilt the box and it tilts. Move the box around and you get to see the final Lego construction from different angles, complete with perspective effects.
Oh, and the computer-generated Lego image is also animated. If it's a garage, the little Lego cars scoot about, if it's a building, the little Lego people are wandering about doing their own thing, "Sims"-style, and if its a tipper truck, the truck drives about the top of the box, tipping stuff.