tender, weak, gradually alternating
冄 centers on a state of softness or lack of strength: something tender, weak, or gradually yielding. The meaning 'gradually alternating' is obscure and may be a rare extension, but it fits the idea of a gentle, wavering quality.
The historical origin of 冄 is uncertain. It may be a pictograph or a simplified form, but no clear component breakdown is established.
The top horizontal line looks like a fragile covering, and the two vertical strokes beneath it seem weak and unsteady, like something tender that bends easily.
For ゼン, imagine a Zen monk gently tending a weak, tender plant: Zen -> ゼン, and the plant's softness matches the kanji's meaning.