skein, tuft, lock, wrinkle
綹 centers on a gathered or twisted bundle of thread or hair: a skein, tuft, or lock. The wrinkle sense extends this idea to a crease or fold in fabric, like a gathered line.
綹 combines 糸 (thread) with 咎, which likely contributes the sound. The character is associated with bundled thread, but the exact historical development is uncertain.
The left side 糸 is thread. The right side 咎 looks like a person (夂) with a mouth (口) and a small mark, as if someone is gathering loose threads into a neat tuft or skein.
For リュウ, imagine a skein of thread being rewound: the sound of rewinding is like 'ryuu' as the thread spins. Rewind -> リュウ, and the thread forms a neat lock.