Foreshortening is part of perspective. Any artist whose style is at all realistic has to have at least a foundation in perspective. Drawing buildings in perspective is one thing, but drawing an organic shape like a hand in proper perspective is a little different.
These hands weren't so hard because the foreshortening wasn't all that extreme. As reference I used a few images out of my huge bank of tennis action shots that I shoot every summer. I always like to observe what the hand that is not holding the racquet is doing during the players motions. You can get some really interesting positions of that hand (if you have the shutter speed set to freeze the motion that is).
One good, old book on how to draw hands is a book called “Drawing the Head & Hands” by one my favorite, though lesser known illustrators, Andrew Loomis.
The book is out of print so look for it next time you’re in a used book shop. I sort of created my own copy from a website years ago. Someone posted a JPG of each page. I downloaded each one, cleaned them up in Photoshop, ran off a copy, and bound it up spiral style.
