If you buy one of their products and decide to upgrade within 90 days you can ship back your product and they apply the price you paid for it to the price of the new one you are upgrading to.
For example if you buy a GTX 260 core 216 and decide you want the 295 you can use the step up program to use the price you paid for the 260 as part of the payment for the 295 all you pay out of pocket is the difference from what you paid for the 260 and the cost of the 295