flutter, shaking or vibrating
襜 centers on the fluttering or shaking motion of fabric, like a garment or cloth moving in the wind.
襜 combines the clothing radical 衤 with 詹, which likely contributes the sound. The character is associated with fluttering cloth, but the exact historical development is uncertain.
The clothing radical 衤 beside 詹 suggests a garment caught in a breeze, its fabric fluttering and shaking.
For セン, imagine the fluttering cloth making a soft 'sen' sound as it shakes in the wind: sen -> セン.