This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 牎 is uncertain. It combines 片 (a split wood or piece) with 怱, but the original semantic and phonetic roles are unclear.
Since the meaning is uncertain, focus on the shape: a piece of wood (片) next to a hurried heart (怱). Imagine a window frame hastily made from wood, but remember this is just a mnemonic, not a real meaning.
For ソウ, imagine a soul (ソウ) peering through a window. The soul's sound cues ソウ, and the window image ties to the possible meaning.