The DESS module reads the code that is in the key that you put into the ignition. It then compares the code to a programmed list contained in the DESS. If a match is found the DESS sends a signal to the computer that the key is an authorized key and allows the Spyder to start and run. The BUDs software is required to program the list of authorized keys into the DESS module, for 2014 & later. For 2013 and earlier the programming method was built into the DESS but requires that at least one of the original OEM keys be available.
The problem DESS plagued a portion of the early 2014 bikes which is when the key coding and DESS system was changed from the previous system. Other Spyders before and after have not been plagued by the DESS problem.
What JC says is true but the likelihood of a thief having the BUDS software and the know how to do what JC says can be done is, IMO, very, very, slim. The much more plausible event would be someone picking the bike up on a truck or trailer and hauling it off to part it out. That is not likely. So, strictly speaking the DESS is not an absolute immobilizer. From a practical and realistic view, it is.