twig, treetop
杪 centers on the slender, outermost part of a tree: a twig or the treetop. Both meanings point to the fine, branching tips of a plant.
杪 combines 木 (tree) with 少, which likely contributes the sound and the idea of smallness or thinness, suggesting the small tips of a tree.
A tree (木) with a small (少) part at the top: the twig or treetop. Picture the tiny, delicate branches at the very end of a tree.
For ビョウ, imagine a twig so thin it looks like a single eyebrow hair: eyebrow -> ビョウ, and the treetop twig is as fine as an eyebrow.