I first saw this video a few years ago when my cousin in Colorado was jumping up and down about it.
The way the rear legs collapsed first, my immediate thought was it was a spinal impact. I would say she's hit the animal somewhere between the T1 and T6 vertebrae, either directly or just below. Looking as closely as you can at the rather lo res video of the impact, it appears to impact high, in line with, or just in front of, the front line of the foreleg.
The behaviour of the animal on impact is very similar to a typical high shoulder shot, which destroys the brachial plexus and instantly incapacitates the animal. There's a bunch of YouTube videos that demonstrate this on animals from roe to elk.
The hilar zone will cause collapse, but it isn't usually quite as dramatic as that. The autonomic plexus nerves primarily control cardiac and pulmonary functions, but still, if you destroy them, it corrupts the CNS as a whole, and down he/she goes. Any direct spinal interaction causes that very characteristic instantaneous flexing of the rear legs and dropping on the rump as the animal falls.
I shot a red spiker at 140m a couple of weeks ago with a 165gr Speer BTSP, and decimated the autonomic plexus. On impact the animal kind of froze and then simply tipped over sideways, quite slowly, as if it was a statue of a deer being pushed over.
Couple of things about that video.
1. She got away with it. It was a clean kill, but the instructor clearly said 'right behind the shoulder'. I think she hit high, and forward, of where she intended. If she had hit behind and low by the same margin, she would have collected the lower rear lungs behind and/or above the heart, and that animal would have taken off.
2. Therefore, if she had aimed at the hilar zone as per the pictures above, if she was high, low, behind or in front of where she intended, she would have inflicted a deadly blow. With a high probability of quick collapse. The hilar zone gives the shooter a wide area of quick kill zone. This is the problem with a "behind the shoulder shot" at long range, if it goes further behind than intended, the rear lungs or worse the liver will be struck, and the animal will run.
3. The Berger VLD is a very poor bullet for this application. There are several long range tests of the VLD Hunting range in various weights, and what has been proven is that once the bullet has lost ~one third of its muzzle velocity, its expansion can be very poor to almost nil. This shot, at 688yds with a .243 at ~2900fps MV means that bullet was impacting at around 1800-1850fps, which is right on the limit of what will generate any expansion. If that bullet had hit behind the shoulder as planned, and between ribs, it would likely have sailed right through with a pencil wound. They require solid muscle mass open up at medium ranges, let along a 700yd shot. It's an all-round dodgy shot on an animal such as elk in every sense of the word, well beyond .243 capabilities, even with a fast twist barrel. The outfitter should have had his arse kicked.
4. My aforementioned cousins shoot custom barreled 1:8" twist Savage .243s, built specifically for winter coyotes. They used 105gr A-Max before, and now use 108gr ELD-M. Accuracy at 600-700yds is exceptional in typical >>> sub-zero, windless conditions. They flatten coyotes with ease. If you took one of those rifles and pointed it at an elk at that range, you'd likely get a punch in the head.
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