crouch
紆 centers on bending or curving in a constrained way, like a thread that twists and turns. The given meaning "crouch" is a specific bodily application of this idea of bending low.
紆 combines 糸 (thread) with 于, which likely contributes the sound and the idea of bending or curving. The exact historical development of 于 in this context is uncertain.
The left side 糸 is thread, and the right side 于 looks like a person bending over. Picture a thread that twists and bends, or a person crouching low like a curled thread.
For ウ, imagine a thread bending into a U shape: U -> ウ, and the thread curves into a crouching posture.