Hemiballismus is due to a lesion in the contralateral subthalamic nucleus (part of the basal ganglia), mostly due to hypertensive crises. It presents with 'wild' swinging of the extremities (contralateral to the lesion).
In the illustration, the subthalamic nucleus (STN) is in yellow.
Source: Wikipedia.

In the illustration, the subthalamic nucleus (STN) is in yellow.
Source: Wikipedia.