to divine
乩 centers on seeking hidden knowledge through ritual: to divine or practice divination.
乩 combines 占 (divination, fortune-telling) with 乚, which may be a simplified or variant form of a component related to the act of divining. The exact historical development is uncertain.
The left side 占 means fortune-telling, and the right side 乚 looks like a bent hook used in a ritual. Picture a diviner using a hooked tool to scratch symbols and reveal hidden knowledge.
For ケイ, imagine a diviner consulting a sacred kay (a made-up ritual object pronounced like 'kay') to divine the future: kay -> ケイ, and the hooked tool 乚 is the kay.