wrap
繃 centers on the act of wrapping or binding something, typically using a cord or cloth.
繃 combines 糸 (thread) with 崩, which likely contributes a phonetic element. The thread radical suggests wrapping or binding with cord.
The thread radical 糸 on the left wraps around the right side 崩 like a bandage, binding it tightly.
For ホウ, imagine wrapping a bandage around a hollow (ホウ) object: hollow -> ホウ, and the thread wraps it securely.
first-aid dressing