This video starts with Part 2 of the fly line cleaning process.
If a fly line gets really dirty then the microscopic pores on its surface start to fill with muck.
Simply wiping the line down with a cloth and soapy water won't get the job done.
If your line is not floating the way it should, or it has become more difficult to mend, this down deep dirt is most likely the culprit.
Although not absolutely necessary, it's a good idea to complete Part 1 of the process prior to performing Part 2.