You cut the first 6-8 rows loose and attach a seperate control rod to that set of rows. Then you can run those rows 1_8-1_4" open more and cut the middle back for a cleaner sample. what you are doing is taking advantage of the big blast of air comming from the fan. The chaff and cob which is lighter than the grain is blown up and out while the grain which is heavier drops down onto the sieve. I have down this to an F,F2 and now my N6 and I really like the job it does. For the rest of you machine click on tips then Gleaner upgrades. lots of good info that will really make you Gleaner Meaner! Enjoy! Pete Hinrichsen