why, how
怎 centers on questioning the manner or reason behind something: asking why or how.
怎 combines 乍 with 心 (heart/mind). The exact historical development is uncertain, but the character likely expresses a questioning state of mind.
The top 乍 looks like a sudden movement, and the bottom 心 is the heart. Imagine your heart suddenly questioning everything: why? how?
For シン, picture a detective asking 'why' and 'how' while examining a scene: scene -> シン, and the heart questions every detail.