What I envision he is suggesting is you have your planned course line from point A to point B. You also have your planned altitude and the winds aloft at that altitude. Using your airplanes specified glide ratio, altitude, and winds aloft, iFly would draw two lines parallel to your planned course, one on each side of your planned course, and impart a colored shading to the area between the parallel lines. This shaded area would be your gliding distance as you fly along your course. The downwind line would be farther away from your course line than the upwind line. Also, as terrain rises, the lines would converge closer to your course line. Seems like a pretty neat idea to me.
You could use this feature for flight planning purposes to drag you planned course line so that more airports are within the shaded area between the parallel "glide distance" lines. During actual flight, if you didn't feel good about having no airports within the shaded area, you could drag your course line one way or the other to encompass more airports. The falicy I see with this is the gliding distance would only be accurate when you are exactly opposite an airport as you pass by.