The brachialis is stretched with full extension of the forearm at the elbow joint, and the forearm halfway between full supination and full pronation at the radioulnar joints.
Note: The halfway pronated/supinated position is done to slacken the brachioradialis and also the biceps brachii as well, so they do not limit the stretch.

The brachialis is stretched with full extension of the forearm at the elbow joint, and the forearm halfway between full supination and full pronation at the radioulnar joints.