Bought one used, no manual. How do I engage/disengage the seeder drive assembly?? The brochure on the their website shows the area on the right side, under the metal cover(?) by the two drive chains in a "distant" photo, no detail of what you move or change. Also doesn't make any sense that you'd have to remove a metal guard held on with two bolts to turn the seeder on or off??????? What the...........?