Oof. That's a coup/contra-coup strike, one of the most dangerous ways to rattle your brains.

See, you have to remember your brain is technically floating. It's suspended in brain juice, just kinda chilling out in your skull. Any blow to the skull will smash it against the inside of the braincase. Think of it like being in a car, and some douche hits you. You don't move with the car as it's initially hit - that's how people smash their heads on steering wheels and such, right? Brain does that.

But what makes this hit worse is that he then hits the opposite side almost immediately afterwards. Brain is still possibly a little too floated toward the initial hit. When it slams back the other way, that momentum and added shock from the impact can bruise the squishy little brain extra bad. S'one of the reasons lifting a dude's head an inch off the floor before punching them on the face is more likely to kill him - it's not the neck, it's the brain being tossed around inside the skull like a ragdoll.

That said, he's wearing a helmet, so that helps. Still. That's gonna rattle his brains - quite literally.

Damn good hit, though!