melt, be charmed, captivated
鑠 centers on a powerful transformation that softens or overwhelms: metal melting into liquid, or a person being so charmed or captivated that their resistance dissolves.
鑠 combines 金 (metal) with 樂, which likely contributes the sound and the idea of melting or dissolving. The character originally referred to melting metal, and the captivated sense is a later figurative extension.
Metal 金 on the left, and 樂 (music/pleasure) on the right. Picture intense heat melting metal until it flows like a beautiful, captivating melody.
For シャク, imagine a blacksmith melting metal with a sharp, rhythmic shock of the hammer: shock -> シャク, and the metal softens under the captivating rhythm.