THere is reasoning about a correct position (thumbs up/hand slightly angled like withers). First it keeps the elbow closed to the trunk, and secondly the bones of the forearm are more neutral. The only negative thing in the first pics it that your wrists are broken upward which is stiffening. When you 'use' the hand it is more like putting a key in a lock, slight turning outward of the thumb will allow the horse to feel a suggestion of an aid w/o pulling back. (And the thumb up position is true whether english or western.) Further the upper arm should hang vertically, straight line from elbow to horse's mouth.
A flat hand is very tensed, and worse yet it would likely be used with a low manner/ and flat (hence action of bit on the bars of the mouth).