Stand up and try to hold your arms out in front of you, with the palms facing straight down.

You'll find that this is a little awkward. The natural resting position of your hands is with the palms facing inwards, not down.

So if I understand right, this image of a T-Rex [1] would be wrong, because its palms are facing downward, while this image of a T-Rex [2] would be right because its palms are in a "clapping" posture?

But I'm still a little confused. Most quadrupeds have their front toes facing forward, right? If the first T-Rex did a belly-flop and caught itself on its palms, they'd be facing forward like a dog's. If the second T-Rex did a belly flop, its toes would be facing outward, like Charlie Chaplin's feet.

1. https://geppettostoybox.com/wp-content/uploads/2021/01/trex....

2. https://s3.envato.com/files/471149443/Realistic%20Trex%20Din...

Oh, that helps me. I thought it had something to do with rotating the palm. Why did they go into all the detail of the ulna & radius crossing?

The counter example they gave was the elephant - but this video [1] of elephants walking looks to me like "bunny hands", at least to a degree.

[1] https://www.youtube.com/watch?v=yf1K63tc1bY

Strangely, for me, downward is more restful than inward. Must be the decades of keyboard use ...

I agree with the article (well, the sauropod tracks in the article) that the natural resting position of your arm as you extend it forward has your palms mostly downward and a little inward. Fully downward is much, much more natural than fully inward.