to drive out demons, etc., (as was done at the new-year)
禓 centers on a ritual act of driving out evil spirits, specifically a new-year exorcism. The given meaning is a single specialized sense, so the core idea is a ceremonial purification or expulsion of demons.
禓 combines 示 (altar/ritual) with 昜, which likely contributes the sound. The character is associated with a specific exorcism ritual, but the detailed historical development of the right component is uncertain.
The left side 示 suggests a ritual or altar. Picture a ceremony at an altar where a priest drives out demons, especially at the new year.
For ヨウ, imagine a priest shouting 'Yo!' to drive out the demons during the ritual: 'Yo!' -> ヨウ, and the demons flee.