odour
飶 centers on a smell or odour, specifically one associated with food or cooking, as suggested by its food-related left component.
飶 combines the food radical 飠 with 必, which likely contributes a phonetic element. The exact historical development is uncertain, but the character is associated with the smell of food.
The left side 飠 is food, and the right side 必 looks like a nose sniffing. Imagine smelling the odour of food cooking.
For ヒツ, imagine a hit of odour from the food: hit -> ヒツ, and the smell hits your nose.