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I know, sir. "A severe injury to the brain at the level of the brainstem is the usual cause of decerebrate posture." But didn't Terri retain good brainstem function? This all adds a few new puzzlements and leads for our plucky medical investigator, doesn't it?

Actually, no. If you look at it in more detail, the brainstem is actually composed of three structures, the midbrain, the pons, and the medulla oblongata. Technical sites describe decerebrate posturing as an injury to the midbrain.

There is a hierarchy of what gets damaged by lack of oxygen and blood supply. Check out this page. Relevant quote:

here is also a regional variation in susceptibility to HIE.[Hypoxic Ischemic Encephalopathy] The cerebral cortex and striatum are more sensitive than the thalamus, and the thalamus in turn is more sensitive than the brainstem. The spinal cord may remain uninjured even when all the rest of the CNS is severely damaged.

and

The most common pattern of injury in HIE is selective loss of sensitive neurons (pyramidal cells of CA1 of the hippocampus, layers 3, 5, and 6 of the neocortex, Purkinje cells, and striatal neurons). Mild HIE, such as a brief cardiac arrest, may affect CA1 pyramidal neurons of the hippocampus only. Bilateral hippocampal damage (see below) causes Korsakoff's amnesia. This is a memory disorder characterized by inability to retain new information (anterograde amnesia) and a less severe defect of recall of old memories (retrograde amnesia). Diffuse cortical, thalamic, or combined neuronal loss (with intact brainstem) results in dementia or the persistent vegetative state (loss of cognitive functions and emotion with preservation of sleep-wake cycles, autonomic function, and breathing).

So for anoxia to cause brainstem injury, means that since the brainstem is relatively resistant, the cortex is already damaged. To have decerebrate posturing, the midbrain is affected, but to breathe, the medulla oblongata is functioning. So there is some brainstem damage, but the “lower” part is functioning.

>> There are several sports that can supply that sort of acceleration.

What is there that sport can do that personal violence can't

Unless someone is using a method to increase acceleration, plenty. Skiing, snowboarding, snowmobiling, car racing, diving, are a few examples of sport that have high speed injury possibilities. Person to person unarmed fighting can’t come close to that kind of acceleration.