stone probe, pierce, counsel
砭 centers on a pointed stone tool used for piercing or probing, and by extension, offering pointed counsel or criticism that pierces through error.
砭 combines 石 (stone) with 乏, which likely contributes the sound and may suggest piercing or probing. The exact historical development of 乏 in this context is uncertain.
A stone (石) needle (乏 looks like a needle piercing) used to probe and pierce, like an ancient acupuncture tool. Its sharp point also represents piercing counsel.
For ヘン, imagine a hen pecking at a stone probe: hen -> ヘン, and the hen's beak pierces like the stone tool.