Thanks for the explanation of keeping arm, wrist, hand, and marker in a straight line. I have been watching the past couple days, and while he doesn’t really seem to hook his wrist, it also doesn’t really form a straight line either. I don’t know if that makes sense, but I have been correcting it to a better line and I also moved the whiteboard to the left side of his body and that seems to have helped as well. I write (right-handed) straight in front of me, but I can see that this doesn’t seem to work as well left-handed for keeping the position the way you described.
Delightful Writing recommends a progression from air writing to cornmeal/rice writing to whiteboard writing to paper writing. I am tempted to skip the whiteboard, but if it is making hand position errors obvious to me, maybe it would be best to keep it just to keep me on the lookout for best hand position.