There is no true answer, really. Only theories.
My personal theory is that their hands shake because of fear, or perhaps as a forewarning of approaching danger. I noticed in the Entire Mystery, thanks to the different angle, Mike’s hand shakes in almost the same manner as Cooper, Pete, Harold etc. after he recreates the sound of electricity (the scene where he says ‘I am the arm and I sound like this”. Listen on headphones, it’s the same noise that plays when the phone polls are shown). Someone on the Dugpa forum also noticed a connection between electricity and the shaking of hands. I think BOB had something to do with it as well. That was the first thing that popped into my mind when I saw only his hand emerge from the shadows later in the series.
I don’t have the time to go through all the topics on Dugpa forum to find the thread I mentioned, because I’m borrowing a friend’s computer, but I strongly suggest you check out this site for interesting facts and ideas.
All I can suggest is checking Google images or taking some screen caps of your own. There are many close-ups in FWWM.
Sorry, Batgrrrl, for some reason Tumblr won’t let me answer you normally.
The first photo is of the symbol as Cooper and the gang found it in Owl Cave. The second is how Earle corrected it. The latter seems to be the true way as that is how Mike (or the Man From Another Place) holds it and how it was placed under the Chalfont’s trailer. Hope that helps.