Re: Keeping the wrists straight.
My friend- Interesting observations from these teachers (in the books, not on this forum thread), but if that's the way they're teaching, they're missing the point, and they're not conveying the true fundamentals.
Firing hand (trigger hand): The LOP and position should be such that the grip upon the pistol grip should have all forces at the foremost portion of the pistol grip. If the LOP is too long, then you'll note that you'll feel forces upon your finger tips, and on the left side of the pistol grip. (forcing the rifle to the right) Thus, shorten your LOP. If the LOP is too short, you'll find that the heel of your hand is digging into the pistol grip, and effectively forcing the rifle to the left. You'll know your LOP and position are both correct when you feel force at the very front of the pistol grip, and that coincides with perfect rearward pressure from a location ~ base of the fingers. So, my advice to you is don't worry about this whole "straight wrist" or "bent wrist" thing. What manifests when you're feeling the pistol grip is ultimately what's important. From there, fix your position or LOP in such a way that the forces upon the rifle from your firing hand are directly rearward (not left, not right).
Left hand (support hand): Given a sling, properly positioned upon the arm and around the wrist/upper arm: The forend rests over the wrist in such a way that the forces of the rifle transmit directly through the wrist and down to the elbow. There are no forces upon the hand, other than compression through the hand. Look at your left hand (really). Place your right hand to the left of the centerline crease of your left hand, and directly above the wrist. Push. Your left hand just rotated to the left. Place your right hand to the right of that same centerline crease. Push. Your left hand just rotated right. The rifle's forces upon your support hand need to be centered over that centerline crease, in such a way that there are NO tendencies of that wrist to break either left or right. Thus, it stays straight. But now you know why, and how.
The whole thumb along or touching or whatever...... BS. Just let your thumb hang out and enjoy the ride. It should have no ability to affect your position. If it does, you're doing it wrong.
Finally, I mean no offense to those others with a difference of opinion. However, I ask those folks to consider the above and challenge me if they think I'm off base. It's too bad that some folks try to get a shooter into an orthodox position, without truly understanding the fundamentals. It's the fundamentals that drive the orthodox position, not the other way around. But then again, I applaud all those who are working hard to train up our next generation, regardless of the pedigree. Thank you to all!