Yesterday, during IFR evening, I completed ground BFR portion with a new-to-me instructor. I liked that he started by asking “what is your mission”? A: >50mi cross country in own aircraft; following which, he optimized our time to relevant topic areas. He stumped me with: “why idle power on the ground is NOT as much of a carb icing risk, as idle power in descent”? Q. what would you have said? (I tried a bunch of familiar to me explanations…)