breast, strike
膺 centers on the chest as a physical and symbolic center: the breast or bosom, and by extension the act of striking or receiving a blow to the chest.
膺 combines 䧹 (a phonetic component associated with the chest) with 月 (flesh), emphasizing the bodily chest area. The strike meaning likely extends from the chest as a target or source of force.
The outer shape 䧹 suggests a bird-like form, and the bottom 月 (flesh) marks the body. Picture a bird puffing out its breast: 膺 is the breast, and striking that breast makes a bold gesture.
For ヨウ, imagine a warrior striking his breast and letting out a bold 'Yo!' as a challenge: Yo -> ヨウ, and the chest is the center of the action.