the kneecap
臏 centers on the kneecap as a specific anatomical part. The meaning is narrow and concrete, with no extended abstract senses.
臏 combines the flesh radical ⺼ with 賓, which likely provides the sound. The character is used specifically for the kneecap, but the detailed historical development of this combination is uncertain.
The flesh radical ⺼ on the left points to a body part. The right side 賓 looks like a guest under a roof with a shell, but here just imagine a guest's knee hitting the table: 臏 is the kneecap.
For ヒン, imagine a hen pecking at a kneecap: hen -> ヒン, and the bird's beak targets the kneecap.