shake, tremble, quake, flutter
擻 centers on rapid, repeated motion: shaking, trembling, quaking, or fluttering. The meanings all describe something moving back and forth or vibrating.
擻 is the modern Japanese simplified form of 擻. The older form combines 扌 (hand) with 數, which likely contributes the sound and the idea of repeated action. The exact historical development of the simplified right side is uncertain.
The hand radical 扌 beside the complex right side 數 suggests a hand causing something to shake or tremble repeatedly.
For ソウ, imagine a hand shaking a saw back and forth: saw -> ソウ, and the shaking motion matches the kanji's meaning.