This gambling effect (also known as "Fast And Loose") is extremely deceiving. Although you are in charge 100% of the time, everything appears fair to the spectator. A chain is placed into the shape of a figure eight. You tell the spectator that one of the loops catches your finger and the other does not. You demonstrate that, indeed, one catches and the other does not. However, even though the spectator has a free choice of which loop is the "catching" loop, they will lose every time (or whatever desired outcome you'd prefer). You could also use this to predict how many time they will win/lose.