It's as Paul said. I'll provide an example to help illustrate...Person A wants to put $10 on Horse XYZ to win at 4.0. Person B accepts that bet. In this example Person A is "backing" and Person B is "laying" the horse. Theoretically Person B is given the $10 at this point (in reality Betfair holds the money until the end).

If the horse wins, Person B has to pay $40 to Person A. This includes the original stake so in this case the Person B has lost $30.

If the horse loses, Person B keeps the $10 stake and in this case Person A has lost his $10 stake.


So to answer your question, the key to making a profit in laying (well in any gambling really) is to find a method that has a strike-rate that performs better than the "value" of the odds at which you are betting. Take the above example (laying $10 on a 4.0 horse) - you would be risking $30 to make $10. So that equates to the same thing as backing a horse @ $1.33 for $30. If you wanted to make money from that you would need to ensure that your strike-rate was above 75% just to break even.

Hope that helps.