Think to your beginning guitar lessons.  I’m sure that you remember being told that your left hand thumb needed to be behind the neck of the guitar.  This works great as a foundation, however just like many other things with technique, as you become more advanced versatility comes into play.  There are three different ways that the left hand can navigate vertically on the fretboard.

  1. Keeping the thumb and left hand stationary, curling and extending the left hand fingers.
  2. Allowing the thumb and left hand to move vertically (up and down), while keeping the same amount of curvature in the fingers.
  3. Pushing and pulling the left wrist, thus causing excess bend in the wrist.

Personally, I use a combination of methods 1 and 2.  I find that this is the most comfortable for my left hand (as it doesn’t involve positioning the wrist awkwardly).  As I navigate up and down the strings, my whole hand will move up and down and as well.  When I get toward either the top or bottom of the neck, I’ll curl my fingers a bit more (traveling to the treble strings) or extend my fingers (traveling to the basses) so that my hand isn’t at an extreme part of the neck (either at the bottom edge or having the inside of my palm touching the bottom of the guitar).  I do occasionally use the 3rd method, but typically it is saved for awkwardly positioned chord voicings or playing past the 12th fret on the guitar.