suddenly, brighten, illuminate, light up
赫 centers on a sudden, intense brightness: something that illuminates or lights up vividly, often with a sense of abruptness. The idea of "suddenly" is tied to this flash of radiance.
赫 doubles 赤 (red) to express intense brightness or a sudden blaze. The repetition of the red component suggests a vivid, overwhelming light.
Two 赤 (red) components side by side create a double dose of fiery red light, like a sudden flash that brightens everything.
For カク, imagine a sudden bright flash that makes a sharp "kak!" sound, like a camera flash: kak -> カク, and the scene is instantly illuminated.