crouch
跧 centers on lowering the body close to the ground: crouching or squatting down.
跧 combines 足 (foot) with 全, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a foot-related action.
The left side is 足 (foot), and the right side is 全 (whole). Imagine your whole foot planted flat on the ground as you crouch down low.
For セン, picture crouching down to pick up a single sen coin: sen -> セン, and your foot stays steady as you lower yourself.