candle wick, lamp wick, stick of
炷 centers on the core of a flame: the wick of a candle or lamp, and by extension a stick-shaped object like incense that burns.
炷 combines 火 (fire) with 主, which likely contributes the sound and may suggest the central or main part of a flame—the wick.
Fire 火 plus the main part 主: the main part of a fire is the wick that keeps it burning.
For シュ, imagine a shoe-shaped candle wick: shoe -> シュ, and the wick burns inside the shoe-shaped candle.