WHen you call up a drawbar organ type sound, the Left Pedal
of the 3 pedals becomes a Rotary Speed control pedal!
(see more info below)

Changing Rotary Speed Using Pedals
The Mark 12's effects unit simulates the sound of a rotary speaker system using the
Rotary setting. Real rotary speakers usually allow the speed to be changed while playing
by flipping a switch, producing an interesting sound that varies over time. Now you can
do this same thing on the Mark 12, using one of the foot pedals to control the speed of the
rotary modulation.
When a sound is chosen which uses the Rotary effect (for example, one of the Drawbar
Organ sounds) the left pedal automatically becomes active as the Rotary Switching pedal.
Press this pedal to hear how the rotary speed ramps up and down. When you switch to
another sound that doesn't have the Rotary effect, the left pedal will perform its usual
function. You can assign any of the three pedals (or none at all) to have this function in
the Preferences menu on the Automatic Rotary Switching page.