This is what I always called the "Oh CRAP" shot...except I use a more color metafor which is spelled like shot, but with an I. Anyway, after whacking at Luke for what seems like hours, all that talking and convincing, and even disfiguring his own son, Luke decides it is better to risk death than become the literal right hand of the evil Darth Vader, so Luke lets go of the pole and falls. And this is the look Vader has on his "face" as he actually fails to convince Luke. A fun draw, and I am always amazed on how movements can help convey emotion even through a rigid mask.