Why is the Helium Flash event in low-mass stars not observable from the star's surface?

Answer

The outer layers are already expanded and opaque, hiding the sudden energy surge

The Helium Flash is a runaway event in the dense core, but the star's already expanded and opaque outer layers prevent this sudden energy surge from being visible externally.

Why is the Helium Flash event in low-mass stars not observable from the star's surface?
astronomystarstellar evolutionmain-sequencered giant