Hello Susan
I understand that you are a Bikram yoga teacher asking for some further tips for your classes. Thank you for asking.
When you are moving your arms in Pranayama I don’t think there is much room for thinking what should be moving in what way. We each have our own degrees of flexibility and movement in the myriad of joints from the shoulders down to the fingertips. So, we really do what we can. Someone with inflexible fingers will have an entirely different envelope of movement to someone with inflexible wrists.
Work the arms up. And rather than worry about curving wrists I would be paying more attention to how the elbows can move up at the same time as relaxing the shoulders down and back. This is far more impactful on the pose and the benefits that can be gained.
Namaste
Gabrielle