spin thread, bring together, shine
緝 centers on joining fibers or elements into a unified whole: spinning thread from separate strands, bringing things together, and by extension, a smooth, lustrous surface that comes from tight, even gathering.
緝 combines 糸 (thread) with 咠, which likely contributes the sound and the idea of gathering or whispering together, reflecting the action of spinning fibers into a single thread.
Thread 糸 on the left, and on the right a shape suggesting many small mouths 口 and an ear 耳 gathered together. Imagine many whispers being spun into one clear story, just as fibers are spun into a single shining thread.
For シュウ, picture a shoe factory where threads are spun together: shoe -> シュウ, and the finished thread shines.