I'm uncomfortable with linking videos of various real-world scenarios of people being shot in the head, but if you search Liveleak, in the Syria section, there's no shortage. Spoiler: they tend to jerk backward and fall. In any event, the video is relevant. The force of an exploding paintball was enough to send that kid's head snapping back like he'd taken a stiff jab. You're arguing having your skull blown out violently couldn't cause a person's head to lurch back a little.
As to your video, it's 10 minutes.... what pig? Not watching the whole thing, so I skipped to the end where the pertinent shit seemed to be. They shot a dummy. The momentum propelled the dummy backward shortly and caused it to fall. No, it didn't go flying... but obviously the momentum of the bullet had an effect. Is that what you're referring too? If not,