Aye it's just the trade out button. If you look at the price of the draw before the match starts, usually it just looks wrong when you compare it with how often a draw actually happens. But the exchange markets often overreact to a good opening partnership and the price of the draw will come in even more. You don't even have to wait for a good partnership if you don't want to. If you lay the draw pre match and there's a few early wickets you're quids in anyway. But waiting a bit allows you to take advantage of the market overreacting. Basically buy low, sell high is the same rule as lay low, back high.
From memory, I think the golden figure was 3.5 which was generally considered a good price to lay at.
If I could zoom in to the graph you could see how much it swings out when there's a couple of wickets, but aye you basically just use the trade out button when you're happy with the profit figure
Obviously you wouldn't try it in a match where there was bad weather forecast.