Very clever! They get you to replace your aff links with their aff link (via plugin - wp) and the customer then go through their link and buy item. The good part is you get a bit more than 6% commissions from the sale.
The bad part is they get their cookie stashed on the customers computer (not yours) and should the customer go back and buy again then they get the commissions on the sale and not you.
So in the case of amazon there is a 24 hour cookie - but if the customer put the item into their cart then there is a 90 day cookie. Loss of a future sales for you!
For some an easy choice and for others not so easy. If you gain a lot of extra commissions through your cookie on the customers computer then this will not be for you.