The general idea behind it is to set up a short-lived variable filter, aka it uses an item to be sorted as a filter to try and pull an item from a double chest above. If successful, it remembers that it was, flushes the filter and puts one item back into the chest above.
With the pre-sorter they added in front of the system, it will group items to do stacks of one item type at a time, with a short delay in between item types.
While there is no such delay, it will just continue flushing items into the storage chests. As soon as there is a short delay between items, it will reset to the ground state, meaning the next item that comes in will be checked against the filter chest again.
Hope that explains the general idea behind it ^.^