to fall from, to fall into
磒 centers on falling downward, especially from a height or into something. The meanings "to fall from" and "to fall into" both express this downward motion.
磒 combines 石 (stone) with 員, which likely contributes the sound. The idea of a stone falling is consistent with the meaning.
A stone (石) beside 員: picture a stone falling from a cliff or into a well, with the right side suggesting the sound of impact.
For イン, imagine a stone falling into an inn's courtyard: inn -> イン, and the stone lands with a thud.