signal fire
燧 centers on fire used for signaling or communication: a signal fire, beacon, or flare lit to send a message over distance.
燧 combines 火 (fire) with 遂, which likely contributes the sound and carries a sense of proceeding or sending forth, together suggesting fire sent as a signal.
The left side is fire 火, and the right side 遂 suggests sending or proceeding. Picture a fire lit on a hilltop, sending its signal far ahead.
For スイ, imagine a signal fire swaying in the wind: sway -> スイ, and the fire's light sways as it signals.