Sharpener chain broke this spring and it has to sharpen at least one pass to set the shear bar so we had to try to use a wrench and turn the motors manually. We had the cutterhead on and would turn one side a couple turns, then the other, then the other side some more............We found out why the motors start reversing as soon as the knock sensor goes off. Knives against the shear bar kept getting louder, and LOUDER, and LOUDER.  I couldn't spin the motors backward fast enough with a wrench and my brother had to run in and kill the engine.  We then backed it off until you could spin it by hand and were really carefull feeding it in forward again. 

Be carefull if you can't sharpen and set the bar manually.  I think there's a part in the operators manual about this procedure.