It's done on Betfair site, I had £200 on MASLAK but as you are allowed to bet during the race, I had also laid the horse to lose at odds of one to five odds on, this is to get my stake outplayed back, if MASLAK were to lose near the line! as in effect that turns out to be a loosing bet, It reduced stake loss, to only £50 if it got caught on the line. I could have layed MASLAK so that all the horses showed say £1500 at the winning post, but this reduces the win money on the winning horse, as you laying the horse that won, so it comes out of your win!