The risk is too your neck but not very likely a risk of concussion.

Personally, I would never let anybody do such a test on me.
The instructions mention the need to not manipulate the vertebra. With this twist, the vertebra has to move quickly and the vertebral artery is contained in that area. A dislodging of plaques in the vertebral artery could cause a stroke similar to a beauty parlor stroke from extending the head into the shampoo bowl.

I prefer videonystagnography using the goggles and computer.