sieve, to sift, to strain
簁 centers on separating fine particles from coarse ones: a sieve, or the action of sifting and straining.
簁 combines 竹 (bamboo) with 徙 (move, shift), suggesting a bamboo tool used for sifting or straining by shaking.
Bamboo 竹 over 徙 (move) — picture a bamboo sieve being shaken back and forth to sift flour.
For シ, imagine sifting flour and seeing it fall like a sheet: sheet -> シ, as the fine powder strains through the bamboo sieve.