Cerebellum
The cerebellum, also known as the ‘little brain’, controls fine motor coordination and equilibrium. These functions can become disrupted under the influence of THC. When you are stoned, it is harder to keep your balance.
Basal ganglia
The basal ganglia (regions of the palaeencephalon, the more instinct-driven part of the brain) are also affected by THC. The basal ganglia are involved in involuntary muscle movements, those you do not have to think about. When you are on cannabis, your body movements are more sluggish and you become more conscious of them.
More details are shown in the animation Drugs and the Brain. It explains how those effects come about.