mortise and tenon, fit into
榫 centers on a woodworking joint: a mortise and tenon, where one piece fits tightly into another. The idea extends naturally to fitting things together snugly.
榫 combines 木 (wood) with 隼, which likely contributes the sound. The character is a specialized term for a woodworking joint.
The wood radical 木 on the left and the right side 隼 (which looks like a bird with a tenon-like beak) suggest a wooden joint: a mortise and tenon fitting together.
For シュン, imagine a woodworker saying 'shun' as they slide the tenon into the mortise with a satisfying fit: shun -> シュン.