written appeal, manifesto
檄 centers on a formal written call to action: a public manifesto or appeal meant to rally people.
檄 combines 木 (wood) with 敫, which likely contributes the sound. Historically, such appeals were written on wooden tablets, hence the wood radical.
A wooden 木 tablet with a striking 敫 message: imagine a manifesto carved into wood, calling people to action.
For ケキ, picture a general banging a wooden tablet and shouting 'Keki!' as a battle cry: Keki -> ケキ, and the manifesto rallies the troops.