to fly rapidly
卂 centers on rapid, darting flight: moving swiftly through the air like a bird or an arrow.
卂 is a pictograph of a bird in swift flight, with the curved stroke suggesting the body and wings and the inner 十 representing speed or motion. The exact historical development is uncertain.
The outer ⺄ looks like a swooping bird, and the inner 十 suggests rapid wingbeats. Together, 卂 is a bird flying fast.
For シン, imagine a bird flying so fast it creates a sharp 'shin' sound as it cuts through the air: shin -> シン.