arrive, reach, gather
臻 centers on reaching a destination or a state: arriving somewhere, attaining a goal, or gathering things together at a point.
臻 combines 至 (arrive) with 秦, which likely contributes the sound. The structure suggests reaching or attaining.
The left side 至 means 'arrive', and the right side 秦 looks like a full granary. Picture arriving at a granary where grain is gathered and stored.
For シン, imagine arriving at a shiny new destination: shin -> シン, and you reach the gathering place.