It is an add on to their 1500 ecodiesel "hot tune". Put the tune in your cart and there is a drop down menu to select several options. For your info, the hot tune is really just the regular economy tune. There is an option for off road tune in the drop down which I know nothing about. I assume...
I have only tested this feature once. It is a little disappointing compared to a true exhaust brake. However, compared to the stock tune without it, the braking is noticeable. It doesn't actually slow you down much on a hill, but at least the truck doesn't run away from you like it does stock...