pivot, door
樞 centers on the central point around which something turns: a door pivot or hinge. The meaning extends naturally to any crucial central mechanism.
樞 combines 木 (wood) with 區, which likely contributes the sound and the idea of a bounded area or container. Together they suggest a wooden mechanism that holds or turns within a frame, like a door pivot.
The left side is a tree 木, and the right side 區 looks like a container with things inside. Picture a wooden pivot box that holds the door and lets it swing.
For スウ, imagine a smooth wooden pivot that swings without a sound: 'smooth' cues スウ, and the door glides silently.