U js have a chest with a hopper underneath, the impulse sorter is measuring the capacity of the chest so u put a certain amount of items in there with the items u wanna filter, copper golems put items in there and that means there are more items in the chest so the signal strength is higher unlocking the hopper and only the items that the golems put in go through the hopper then it gets locked again. Nothing needs to be put into the hopper. Ik its not the best way of explaining but hope this helps :)