This is a subtle distinction. A recent study found that people with persistent whiplash couldn't move their necks as much in several directions as those who didn't have whiplash. However, the patterns or kinds of movement weren't all that different between the two groups. This means that, according to this study, whiplash doesn't cause abnormal movement in the neck. It simply restricts the amount of movement that can be comfortably done.
Exactly how whiplash restricts movement has yet to be determined. Researchers think whiplash affects movement by way of pain or changes in the tissues. There may be other factors at work as well.