First of all, I have to thank David Hone, David Unwin, and Michael Skrepnick for helping me to understand this concept, as it was very difficult for me.
I am a pterosaur fan. I think the group is incredibly interesting, and I have devoted several posts to them in the last few years. I try to keep up on my pterosaur literature, but it's tough in Alaska. I try to nab any pterosaur books I become aware of, most recently David Unwin's spiritual sequel to Wellnhofer's Prehistoric Flying Reptiles, Pterosaurs from Deep Time. It is an excellent read, and it brought me up to date on many aspects of pterosaur anatomy that I was only vaguely aware of before reading it.
One of the areas I've constantly struggled with, however, is how the wing folds. More specifically, how the joint between the wing metacarpal and the first wing phalange operates. I've seen plenty of reconstructions, few of which agree with each other, of how the wing folds up while on the ground, and it's just confusing. Without a 3D model, imagining the process is strenuous, but I have put all the lessons together and tried to make an accurate diagram of how the finger joints operate (above).
If you want a human equivalent, here we go:
Put your arms out to your sides, slightly bent at the elbows, palms facing up. Now chop off your pinkie finger--you won't need that. Also, reorient your thumb so that it's no longer offset. Curl your now injured thumb, index finger, and middle finger. Notice how they all curl toward the palm. Great. Now here's the painful part: Dislocate your ring finger, spin it 180 degrees, and pop it back in place. Your ring finger will now curl toward the back of your hand. Ouch, right? Keep your palms up! When you curl that ring finger, notice that it starts pointing toward your elbow.
That's basically the situation in pterosaurs. While in flight, a pterosaur's palm faces forward, and its "ring" finger is bent slightly back. The manual digits--short 'n' scrawny by comparison--were kept in a relaxed position, probably slightly curled inward, toward the palm.
But now the pterosaur lands, and starts walking on all fours. What did the wing look like then?
Don't worry--things don't completely go to hell. You can still use your own arms as examples.
Get down on all fours (knees on the ground), and sprawl your forelimbs out to the sides, so that your elbows and fingers are pointing away from the body. Now rotate your wrists slightly so that your fingers are actually pointing slightly back. Remember that your pinkie finger has been severed (hope you've got some gauze on that wound!) and your ring finger is backwards.
Bend the base of your reversed ring finger "up," and notice how your ring finger now points toward your elbow again. In a pterosaur wing, the individual wing phalanges were virtually unable to move against each other, so aside from the metacarpal/phalange joint, your ring finger is a straight rod. Now imagine that your ring finger is just a bit longer than your entire arm. Whallah! Pterosaur wing!
Of course, if you want to go...you know...all out, you could try and keep your palm off the ground, because of course pterosaurs walked in a digigrade manner for their forelimbs, but plantigrade hindlimbs!
So, please tell me if I've gotten it right, folks! I apologize for using overly violent examples for a human analogue, but all the attempts I made to draw a human analogue ended up looking entirely too disturbing.