The Downward Arm Attack is commonly known in Sport Karate as a Lower Gate Block. There are many uses for this technique, one of which is shown here.
Charlie grabs my left wrist.
I twist my left hand in a circular motion and grab his wrist as I step back and root to the ground. I start coiling and pull Charlie off balance by pulling my hand to my hip (this is a strong body position). My right arm is placed on a pressure point on the Triceps area. This point is called TW-11.
Also notice that my right leg is turned so I can take advantage of hip power by snapping the leg to the front. As I move my right hand up Charlie's arm slightly to pressure point TW-12, a breaking point, I snap my leg to the front and then pull his arm slightly up with my left and snap my right arm down on an angle breaking the capsule in his elbow.