As Mark says, you have first have to ensure the market settles so that you've got a good idea what the S.P. prices will be.

When he is satisfied, I believe he then lays every horse on S.P. for the same amount. S.P automatically "dutch-lays" them all, and if the market is more than 100% its then difficult to lose.

He might identify suitable situations/factors to lay them. I haven't looked at it yet so I'm not certain what he does.