Circuits control how much collectors and crushers create. The belt loop contents are read at all times. The constant combinator outputs the amount of each item you want on the belt but as a negative number. The decider combinator says if input signal less than 0 then output that item as 1. Both combinators are connected to belt reader. So if total > 0 then item is needed. All machines and inserters that need this logic are connected via the green wire.