What is the Leslie Effect? Is it only on the WK3200 and WK3700?

Leslie Effect is basically a "vintage" sound effect. It was derived from Leslie speaker cabinets. When musical instruments, like an organ, were played through this speaker, a distinct sound was produced. This sound can now be reproduced on some Casio keyboards.

This feature is found on all WK3*** models as well as on the CTK691 and CTK900.