curve, stretching a bow
彎 centers on a bending or curving shape, like drawing a bow to its full arc. The idea of stretching a bow is a specific application of that curved form.
彎 combines 䜌 (a phonetic component suggesting winding or tangled) with 弓 (bow), together expressing the idea of bending or curving like a drawn bow. The exact historical development of 䜌 is complex, but the phono-semantic structure is broadly accepted.
The top part 䜌 looks like a tangled, winding thread, and the bottom 弓 is a bow. Imagine pulling the tangled thread to bend the bow into a deep curve.
For ワン, picture a bow bending into a perfect one-curve arc: one -> ワン, and the bow stretches into that single smooth curve.