If you have the (3) pedal unit (purchased separately), then the LEFT pedal will automatically be assigned to control Rotary Speed.
But, if you're only using a standard sustain pedal plugged into the AUX PEDAL input on the back of the DGX; then, you'll have to assign that pedal to control the Rotary fast/slow speed. Here's how to reassign the pedal function:
MENU -->CONTROLLER --->ENTER
On the screen you will see (4) rows labeled 1 Aux: xxxxx, 2 Right xxxxx, 3 Center xxxxx, 4 Left xxxxx.
#1 is the row that assigns what the pedal will function when you plug a pedal into AUX PEDAL input.
I'd you have the optional (3) pedal unit, then:
#2 is the row that you would assign what function the RIGHT pedal will control.
#3 is the row that you would assign what function the CENTER pedal will control
#4 is the row that you would assign what function the LEFT pedal will control.
On the bottom left-side of the window, you will see an up/down arrow above the number 1.
Those Up/Down buttons are used to scroll through the menu to decide what "function" you would like each of the 4 pedals to control.
On the bottom right-side of the window are up/down arrows to turn on/off the pedal for each of the PARTS (i.e. LEFT/MAIN/LAYER.)
NOW: to reassign your pedal from Sustain to Leslie Speed Control:
If you're only using a single standard pedal plugged into the AUX PEDAL input, then use the primary up/down/left/right pad (located below the MENU button) to scroll and highlight Row #1. "AUX: xxxxx"
Then, use the up/down arrows (bottom left-side of window) to choose the function: "MODULATION ALT".
Note: I realize that in the scroll menu, you will see a function called "Organ Rotary Slow/Fast". You would think that function would be the function that actually is used to control the Fast/Slow speed.....but on my DGX670, it doesn't....I have to use the function "MODULATION ALT". Maybe, your DGX670 has a different firmware and you actually use the function "Organ Rotary Slow/Fast" to control the Leslie speed. You'll just have to experiment and see what works for you.
