to light
焌 centers on the act of igniting or setting something alight: to light a fire or kindle a flame.
焌 combines 火 (fire) with 夋, which likely contributes a phonetic element. The exact historical development of 夋 is uncertain, but the character as a whole expresses lighting a fire.
The left side is fire 火, and the right side 夋 looks like a person bending down with a tool. Imagine someone crouching to light a fire with a spark.
For シュン, picture a sudden 'shoon' sound as a flame ignites: shoon -> シュン, and the fire catches instantly.