roast, burn
煬 centers on applying intense heat to something: roasting or burning, typically with fire.
煬 combines 火 (fire) with 昜, which likely contributes the sound and may relate to brightness or heat. The exact historical development of 昜 is uncertain.
Fire 火 on the left and 昜 on the right: picture a bright fire roasting something over an open flame.
For ヨウ, imagine a yo-yo being roasted over a fire: yo-yo -> ヨウ, and the fire burns it.