ANSWER: The rear rotisserie burner is supposed to be either ON or OFF. You can reduce the gas flow by adjusting the knob to a location between ON and OFF. Be careful that the burner does not extinguish from lack of gas.
If the direct IR energy is too much, than you can turn it off at a certain point and, using the BBQ Tray accessory filled with water (to prevent warping and to add moisture to the food), you can use the indirect method of heat. Placing the BBQ tray over one of the burners (and under the meat on the rod) converts its IR energy to convected heat.
In any rotisserie cooking, be sure to cook long enough to achieve the recommended internal temperature, as determined by an instant read thermometer. Recommended temperatures for various meats may be found at http://www.fsis.usda.gov/is_it_done_yet/