Glyn,
With H&S's DVD/Nav in motion, it unlocks all the features of the radio that otherwise are unavailable once you put the truck in gear.
Instead of having to stop to enter any of the nav functions, you can actually use it lime it should be. Ya know, like if you have a passenger they can actually enter nav instructions.
Now, the DVD in motion option.....gotta say, I'm guilty of having a movie playing going down the road. I HIGHLY recommned not doing this....I know I was all over the road.