breath, life
縡 centers on the fundamental force that sustains living things: breath and life itself. The two given meanings are closely linked, as breath is the most immediate sign of life.
縡 combines 糸 (thread) with 宰, which likely contributes the sound. The exact historical development is uncertain, and the connection between thread and breath/life is not clearly established.
Picture a delicate thread 糸 that represents the fragile line between life and death. The right side 宰 suggests control or management, as if breath is the thread that manages life.
For サイ, imagine a sigh of breath escaping as you tie a thread: sigh -> サイ, and that breath is the thread of life.