Another gem of a feature beautifully explained in the owners manual....NOT!
I have used the "RES" button quite often as part of highway driving, so here is how I think it works.
I assume that you all know how the cruise control system is activated. Well once you are in cruise mode, there are only 2 ways of dis-engaging the cruise control:
1. Manually Turn the cruise control off via the cruise control column.
2. Press/Tap the brake pedal.
The second option is very common in highway driving in case you come close to another car or for whatever other reason. In this instance, you can just press the "RES" button to re-engage the cruise mode at the previous set speed.
However, if you manually switch the cruise control off via the stalk, the system "forgets" what the previous set speed was. As such if you switch cruise control to "ON" mode via the stalk, and press the "res" button, nothing will happen. That is, the system will not re-engage the cruise control.
Reading back on this, I find it difficult to understand, but anyways, thats my explanation and I'm sticking to it.