Shinto purification ceremony
禊 centers on a Shinto ritual of cleansing and purification, often involving water, to remove impurity or sin.
禊 combines 示 (altar, religious ceremony) with 契, which likely contributes the sound and may relate to the idea of a pact or binding, suggesting a ritual act of purification.
The left side 示 (altar) and right side 契 (pact) together suggest a sacred pact of cleansing: imagine a Shinto priest performing a purification ceremony at an altar, binding the impurity to be washed away.
For ケイ, picture a purification ceremony where everyone forms a neat queue (ケイ) to be cleansed: the orderly line cues ケイ while the ritual purifies.