anything. When the defender wraps around Mahomes' neck it does two things.

1) stabilizes the cervical spine in a bad way. It's kind of like taking the crumple zone away from a car. All the ligamentous give is taken away. It in effect removed a shock absorber.

2) By lengthening the cervical spine, the cord pulled the brainstem into the foramen magnum and even into the top of the cervical spine. In short it briefly created a tethered spinal cord syndrome.

Then with both of these factors in place, the head was accelerated into the turf. When the head hit, no force was taken up by the C-spine and the brain also couldn't move freely within the skull and be protected by the dura mater.

It was a bit of a freak thing. Not sure what rule changes would help short of requiring all tackles to be below the shoulders.