The customer is taken through a series of offers. After someone buys the Front End (FE) product they will be taken to the OTO/Upsell page. If they do not purchase the OTO/Upsell they are taken to the Downsell page.
If they do not buy the Downsell they are taken to their purchase. If you do not have a Downsell and the customer says no to the OTO they are also taken to their purchase.