This knot is used to join two lines of different diameters.it is used to attach a heavier  'shock leader' to a lighter main line. It requires more attention when forming  the loops to make a neat finish .If you carefully melt the trimmed ends it will reduce the chance of damage to your thumb  when casting, also  helps prevent weed from catching on the knot. 

Blood Knot         Clinched Knot         Grinner