You press 4th selection, 2nd selection, 3rd selection, 1st selection and get the menu and then it gives you credit. But you can't get money back and it only works on older british machines.
I've made a couple of fake £10 before for a laugh, and managed to buy something The trick is to draw the watermark faintly on a peice of paper along with a black line and when you hold it up, depends if you have the right paper, it shows as a watermark :8