one who preceeds king in procession
蹕 centers on the role of a person who walks ahead of a royal procession to clear the way. Its meaning is narrow and ceremonial: a herald or forerunner for a king.
蹕 combines 足 (foot) with 畢, which likely contributes the sound. The character is rare and its exact historical development is uncertain.
The left side 足 means foot, and the right side 畢 looks like a complex ceremonial tool. Picture a foot stepping ahead with a ritual implement to clear the path for a king.
For ヒツ, imagine the herald clearing the path with a sharp hiss: hiss -> ヒツ, as the foot moves forward to make way.