shoe, sole of a shoe, magpie
舃 centers on footwear, specifically a shoe or its sole. The magpie meaning is a separate, unrelated sense that does not connect to the shoe idea.
舃 is a variant form of 舄, which historically depicted a magpie and later came to be used for a type of shoe, possibly through phonetic loan. The exact development is uncertain.
The top 臼 looks like a shoe last or mold, and the bottom 灬 suggests the heat used in shaping a sole. Picture a cobbler pressing a hot sole onto a shoe form.
For シャク, imagine a shoemaker shaking a finished shoe off the last: shake -> シャク, and the shoe drops free.