The different toothed sprockets top and bottom slow down the gathering chain speed which is supposed to reduce trash intake and cause less breakup of the stalk pulling it more through the rolls instead of breaking the top part of the stalk off and sending it through the combine. 6 tooth drive sprocket up top and 9 tooth idler at the bottom. It's part of an entire system by Calmer to reduce trash intake and keep more ears in the head. You can read all about the entire system here. http://www.calmercornheads.com/trashReduction.htm | 
Question on updated, lower 9 tooth cornhead sprocket
