shine, melt
爍 centers on intense light and heat: shining brightly or melting something with that heat. The two meanings are connected by the idea of fire making things glow and soften.
爍 combines 火 (fire) with 樂, which likely contributes the sound and reinforces the idea of intense heat causing shining or melting.
Fire 火 on the left and 樂 on the right: imagine a blazing fire that makes metal shine and melt into a happy, flowing state.
For シャク, picture a shack glowing in the dark as firelight shines through the cracks: shack -> シャク, and the fire melts the frost on the windows.