The Rotary Speed Control is an effect designed to emulate a rotary speaker device. Such a device became very popular for use with an American tonewheel organ around the early 1940s. The original intent of such a speaker device was to create an amplification system for the American tonewheel organ so that it had a better emulation of a theater organ or pipe organ.
The rotating speaker device is a speaker and amplifier combination where the horn device (high frequencies) and the woofer device (low frequencies) have motors which can rotate at a constant speed to effect the sound. At the height of its popularity, this speaker had two speed controls, slow (chorale) and fast (tremolo). The sound created by this amplification system quickly became a signature sound for jazz organists, and soon thereafter became a common fixture in recording studios and within the lexicons of gospel and rock music.
This process is emulated in the Reface YC with a FAST and SLOW setting, as well as a STOP, and OFF setting.
For switching the rotary speaker speed on the Reface YC, moving the lever up or down changes the setting between "FAST" (fast rotation), "SLOW" (slow rotation), "STOP" (no rotation), and "OFF" (no effect applied).
Below is an audio demo of the Rotary Speaker Effect starting out FAST, moving to SLOW, and then to STOP.
This is a STEREO effect and is most effective with both outputs amplified from the Reface YC. You will notice that when you switch to the STOP position, the sound of the Reface YC will stop rotation movement within the stereo field in the position where you activated it to stop.
When the Reface YC is connected via MIDI to a controller keyboard with a modulation wheel (MIDI controller#1), the Mod wheel with change the ROTARY SPEED setting between FAST and SLOW.