I can't stand watching anymore of Cassel & from that angle I don't see the hit doing much damage, but...
I took a pretty big dirt nap in a motocross race a few year back. I remember my chin guard hitting the bars and everything going black. I "woke up" several yards away from where my quad met my face when the back of my helmet hit the dirt. My body ate up 90% of the crash but the back of my helmet hitting the ground as I came to rest is what ****ed me up the worst. As I tried to gather my thoughts I had the same confused look on my face as Cassel clearly has as he sits up. It literally feels like your brain is spinning in your skull. If he's acting he did a pretty damn good job of getting the last part right.
It doesn't take much of a blow to the back of your head to really mess a guy up.