As the author states:
"The idea was to have a treaded robot that could navigate varying terrain, turn quickly and of course, climb. Based on my experience with my other robots using the same tracks (eg UNV and DynaTrax), I found that they were not very good when it came to inclines. I figured that the LEGO rubber wheels have great traction on most surfaces, so why not slap a set of them along with the treads."The point with the DualGrip is that it switches from tread drive to wheel drive when it encounters an incline - pretty good idea, isn't it?
Have a look at the three web pages associated to the robot - there's a lot of information on the DualGrip to be found, including two more movies.