Example

Back something at $3.00 for $100.00
Lay it at $2.00 for $150.00

$50 profit no matter what the result.

Or

Back something at $3.00 for $100.00
Lay it at $2.00 for $100.00

Win $100.00 if it wins, break even if it loses.

So am i backing, then laying when they score or something?

Generally or if you can see something which is overs you may even be able to do it before the even has started.