Rino Masic doesn't include pronation in his top 5 favorite excercises.
Top 5 Best Exercises for Arm Wrestling | Arm Wrestling Training
He includes an exercise for the following:
1 - Bicep
2 - Hammer
3 - Wrist
4 - Rise
5 - Fingers
I'd have to then conclude that number 6 would be pronation. This is pretty controversial is it not? Unless he's corrected himself.