feather (arrow)
矧 centers on the feather or fletching of an arrow, the part that stabilizes flight.
矧 combines 矢 (arrow) with 引 (pull), suggesting the feathers that are pulled along the arrow shaft to stabilize flight. The exact historical development is uncertain, but the component relationship is plausible.
An arrow (矢) needs feathers pulled (引) along its shaft to fly straight. Picture the fletching being drawn tight against the arrow.
For シン, imagine the thin, sharp sound of an arrow cutting through the air: shin! The shin sound cues シン as the feather stabilizes the arrow's flight.