thread
綅 centers on thread as a material or a single strand used in sewing or weaving.
綅 combines 糸 (thread) with a phonetic component that likely contributed the sound. The exact historical development of the right-side component is uncertain.
The left side 糸 is thread. The right side looks like a cover 冖 over a hand 又 holding something small 小, like a needle pulling thread through fabric.
For シン, imagine a thin thread shining under a light: thin -> シン, and the thread gleams as it passes through the needle.