I was asked this by one of my students last night.... and I didn't know the answer which is highly unusual for me. So I've set the student the task of looking into this and I thought I'd ask the experts of this fine forum.
Consider the elbow strike of pinan yondan, tekki shodan, kushanku and others. For the sake of this example so as you can picture what I'm asking it's a right-arm elbow strike. I reach or control the enemy's head with my left hand and strike my left palm with my right elbow. What orientation is my right fist in? And, does it matter?
I always perform it with my fist verticle so as the back of my right hand meets my left forearm. Another option would be to have the right hammerfist meet the left forearm.
Rotation of the fist will cause pronation or supination of the foream and so different aspects of the right foream will meet the left. But I can't see that it makes any difference to the striking point, which is the proximal ulna / olecranon.
Your thoughts are welcomed.