A good way to use this is to use the lovefilm trial and say that someone (if you want to use me im firstname.lastname@example.org) referred you to get the trial up to 30 days. You can cancel before you are charged and it will still count as you doing an offer.
It works like freeipods and all that jazz from the US. Basically companies offer money if they get peopel signed up to thier offers/acocunts etc…
This site collects that money and uses it to buy people items if they do an offer and a number of thier friends do the same.
Here is a wired.com article about freeipods.com. Maybe that will explain it better.