The trick is that you braid a dollar bill. Close examination reveals that it has two lengthwise slits that extend nearly to the ends of the bill, dividing it into three bands. The braiding is real although the bands have never been separate. It looks impossible.
The bill can be borrowed, transformed into this impossible object, and returned to the owner in less than two minutes. The result is a distinctive memento. It is, as Michael Weber says, "A bill that cannot be spent."