pound (mortar), sink, set (sun)
舂 centers on the action of pounding or crushing something in a mortar. The sinking and setting-sun senses are extended uses that describe a downward, pressing motion like the sun sinking below the horizon.
舂 is believed to depict a pestle (the top part) being used to pound grain in a mortar (臼 at the bottom). The modern form retains this basic structure, though the top component has been simplified.
The top part looks like a pestle being held, and the bottom 臼 is a mortar. Picture someone pounding grain in a mortar: the pestle goes down, just like the sun sinking.
For ショウ, imagine a show where a performer pounds grain in a mortar with a dramatic flourish: show -> ショウ, and the pestle comes down hard.