Just browsing climbing falls (as you do), this seems interesting. A sizeable fall from a slab into the air, his leg doesn't catch on anything, but he gets a hell of a nasty catch. I'm wondering if falling down a slab gives you more of an "away from the rock" direction of travel, meaning you get a harsher change in direction when the rope goes tight.
The belayer has probably taken as much slack as possible to avoid a ground fall (understandable), but would a more dynamic catch have reduced the "whip" effect?
Perhaps it's related to the pitch of the climber after leaving the rock, while falling down the slab he kind of ends up leaning back a bit, which sets you up for back pain when the rope tightens.
Post edited at 17:18