Betting on both outcomes of an event in order to make a minimal loss and then aquiring a promotional "free bet" which you can then do the same with in order to make a profit. It's a bit hard to get your head round but it's a completely risk free way of making a few bob. You need an starting pot of about £100-200 to make it worthwhile though.
Looks like I've done it right. WBA losing loses my bet and wins my lay, meaning I come out with £19.21 out of £20, then I can use my free bet on another game in order to get about £17.50 out of a tenner input, regardless of result.