petal, valve
瓣 centers on a thin, flat piece that opens and closes or separates: a petal of a flower or a valve in a mechanism. Both senses share the idea of a segmented, movable flap.
瓣 combines 辡 (historically related to distinguishing or dividing) with 瓜 (gourd, melon), likely suggesting the segmented, separable parts of a gourd or flower. The modern meanings of petal and valve extend this idea of divided, flap-like parts.
The outer 辡 looks like two halves facing each other, and inside is 瓜, a gourd. Imagine a gourd split open into petal-like sections, each one a valve that can open and close.
For ベン, picture a bent petal on a flower: bent -> ベン, and the bent shape reminds you of a valve opening.