RE: Double lung shot
I think it's partly the adrenaline.
I hunted with a buddy in Alabama who put a Bear Razorhead through a perfect typical 12's heart. The buck went for just under 300 yards, having crossed a creek we had to use a canoe to get to the area we hunted. While we were trailing him, he had went down once only about 40 yards where he finally piled-up. The broadhead had sliced a good 1.5" off the tip end of the heart.
Going that far after a heart-shot, or living so long after a double-lung'er. Must be a strong desire to live!?
Shoot often - Hunt always