the slam. That prevents movement and the head takes all the force. To me it looks like the defenders arm slid under the face mask and when the crown of mahomes helmet hit the ground without any give his head took all the force. Doesn't look brutal but his whole body bounces off the ground essentially after the head takes all the force. Doesn't look violent but with no give it's a LOT to absorb. Look up Gerald Harris v Dave Branch for a good idea of what I mean.