My experience is mostly with sporting index. Basically how it works is the more your bet wins by the more $$$ you win. On the flipside the more your bet loses by the more $$$ you lose. So for example backing Manly last night u would be in a penthouse right now and the Warriors in a homeless shelter. Exaggerated (unless u bet reasonably big) but I'm sure u get my point
There some other rules you can take advantage of that are different to normal sports books and other ways you can get burnt. It pays to read through all their rules first.
Just as an example of my own for awhile I obtained info on golf players having injuries. Under their rules if a player W/D's during a round you win big. for a few weeks one of my key capping methods was to back against players vulnerable with injuries (and capped the matchups as per normal) after three weeks Rolf Muntz W/D after 4 holes and the rest is history.
After that I got my account closed as they didn't want my business.
Its a dangerous game If you don't do your preparation first, for people starting out some will offer what is called a shield account to help u learn the ropes. This means there is a retainer on how much u can win or lose. Highly recommend starting this way.....If they don't offer it go to one that does.