A pin on the gear closest to the chuck en is pulled to free it from the pulleys on the spindle shaft. Then the gears on the back gear shaft are pulled into contact with the gears on the spindle. That done, the original belts on the original pulleys drives the back gear by; first the small gear on the spindle shaft, that small gear on the shaft is driving the large gear on the back gear shaft. That large gear on the back shaft rotates the small gear which now drives the spindle which had been disconnected by pulling the pin from the big gear on the spindle shaft which was formerly driven by the pulley system.
To reset, pull the back gear out of mesh. Then slowly rotate the spindle by hand while pushing on the pin in the big spindle gear.. When it is seated, you are back to full speed.
Look at the pictures:
back gear information and photos
~Richard