People with autism often have repetitive patterns of movement.
Here are some common ones:
Pacing back and forth
Flapping their hands
Rocking back and forth
These are often called self-stimming behaviors. This means a person is giving themselves the sensory stimulation that they need. This comes from how the autism affects their experience of sensory information.
Often these are things that soothe the person when they are stressed.
There is nothing wrong with these behaviors unless the action is hurting them. This does happen sometimes.
In this case, you will have to talk to their doctor and therapists to find a solution. For example, kids who bang their head may have to wear a helmet. Kids who chew on their wrist may have to wrap it in a cloth or learn to use a rubber chew item instead.
(There's even a line of "chewelry"! Jewlery you can chew! Click here to see some samples.)