Power Chrod Thumb Position

Trying to wrap my head around this...

I'm working on power chords and I'm getting some wrist pain. So after some observation, I realized the wrist pain is coming from my pushing with my thumb up against the back of the neck. As a result i tend to bend my wrist... I guess I'm using my thumb to help with hand placement accuracy? If I rest my thumb length wise along the neck facing towrads the headstock, I feel like the neck floats.... Just wondering what the proper hand placement should be? Sorry about the blurry picture. It's early. :D

https://photos.app.goo.gl/Vz5aobjwBJT8jZz28

Your finger & thumb position look okay there.  But the more likely culprits are not shown in that image.

First potential problem:  it's difficult to see the angle of your wrist.  You want as smooth and natural a curve as possible.  The more your wrist bends the less power & control you can manage.  So you might want to tilt the neck up a bit more in order to make your wrist bend less.  Also, is that a watch?  I typically discourage anything on your wrist that might impede motion, strength or dexterity.

Second problem:  a lot of hand, fringer, wrist, arm pain comes down to using too much pressure.  Beginners often squeeze very hard.  This is a completely natural reaction because playing is difficult at first!  But as you gain more strength, dexterity and control you want to back off on the amount of tension you use to fret notes.  You only need a certain amount of pressure to make the notes sound.  Any more is wasted effort that hinders your playing.

Hope that helps!