Holding the guitar: Left (fretting hand) hand position and technique

Here is a post from the Facebook student page from adult student Jeff Ortel, who has been pondering left hand technique:

Jeff Ortel
Time to share another jortel guitar revelation: This comes with the usual disclaimers. First, that most of you probably already know this. And, second, that Blake has told me this 1000 times and I just didn't listen :) Thumb position REALLY matters! Lately, I've been feeling like the flexibility and dexterity in my fretting hand really needs work. So I hunkered down this week and reintroduced myself to Chromatic Scales. Ugh! My flexibility and dexterity seemed even worse then I thought. Then, I noticed something interesting. That by changing my thumb position (even slightly), I could magically reach strings/frets that previously I couldn't and my fingers could function more independently. Wow! Not palming the neck and paying attention to thumb position made a huge difference. So then I decided to apply this "magic" to fretting chords. Wow, again! It made some chords feel a bit unfamiliar at first but after few reps, that feeling seemed to start melting away. So, feeling like you're lacking dexterity, flexibility or finger independence? Like your hands are too small? Take a look at your thumb position.