Turn over the cards and show all four have a mirrored back design on them. As you say its all done with mirrors. This will fit in any pocket. I have not seen that before. Do not be put off by the low price this is very nice quality and works very smoothly.
Any note is borrowed, a small aluminium disc is passed out for examination.
The disc is laid on the note. You rub the disc across the note, slowly it starts to penetrate the note, you can actually show the other side of the note with the disc half way through it, this looks really weird. Next the disc pops right through the note. Both the disc and the note can be immediately passed out for full examination. You start off with a blank paddle and then an arrow appears on one end.