to separate the grain from the husk
臿 centers on the action of separating grain from its husk, a specific agricultural processing step.
臿 is thought to depict a pestle (干) inserted into a mortar (臼), representing the action of pounding grain to separate husks. The exact historical development is uncertain.
The top part 干 looks like a pestle, and the bottom 臼 is a mortar. Picture pounding grain in the mortar to separate the husk.
For ソウ, imagine the sound of grain being pounded: a rhythmic 'sow, sow' as the pestle hits the mortar. Sow -> ソウ.