soak through, infiltrate
渗 centers on liquid moving gradually into or through something: soaking through a material or infiltrating a space.
渗 combines 氵 (water) with 参, which likely contributes the sound and the idea of gradual penetration or mixing in.
Water 氵 on the left and 参 on the right: picture water slowly seeping into a porous material, soaking through layer by layer.
For シン, imagine a thin liquid seeping through a thin fabric: thin -> シン, and the water infiltrates the cloth.