How did the hiding place work?

A false was was built in the upstairs of Corrie's home. Construction workers build a brick wall, which is then painted. Bookshelves provide further comouflage. The wall can be moved, so that those hiding can go behind it. When closed..... the room looks just like a room. Smit, the man who creates the secret room, promises Corrie that no one will be able to detect it.