curve, stretching a bow
弯 centers on a bending or curving shape, especially the act of drawing a bow. The idea of stretching a bow naturally involves a curve, so both meanings connect through the image of something pulled into an arc.
弯 combines 亦 (a phonetic element with a sense of bending or curving) and 弓 (bow), together suggesting the curved shape of a drawn bow.
The top part 亦 looks like a bent shape, and the bottom 弓 is a bow. Picture pulling back a bowstring until the bow curves into a deep arc: 弯 is that curve.
For ワン, imagine a one-of-a-kind curved bow: one -> ワン, and the bow bends into a perfect curve.