When something is said that is so complete, clear, clever, and simply so perfect for the situation that it makes you want to shout for joy.
While not strictly limited to work, certainly the most common occurrence of a Pants Down Explanation occurs in the work place.
BOSS: "Jenkins, why are we exhibiting at the Business Expo this week?"
EOMPLOYEE: "Because a 600 of the largest buyers of our products will be in attendance. We've pre-marketed to all of them and have face-to-face pitch meetings pre-arranged with 40. By closing an average size deal at our average selling price with just 2 of them, we will have return on our investment in excess of 500%."
Now that's a Pants Down Explanation.
See http://pantsdownexplanation.blogspot.com/ for more.