These things are (were?) made by Leslie, which is why they are also called 'Leslies'. Usually they are a two-way system, with the low-frequency speaker mounted in the bottom of the cabinet, firng upwards into a rotating ported baffle, and four high-frequency horns mounted on a rotating post in the upper part of the cabinet. The horns rotate in the opposite direction to the low-frequency baffle. These were originally intended for use with electric organs, with the on/off and speed controls mounted at the keyboard.

To get an idea of what they sounded like, get some early Deep Purple, like "The Book of Taleisyn" turn it on, and stand back! A Hammond B3, a 100-Watt Marshall head, two Leslie cabs, and John Lord at full-bore was an awsome experience.