From another thread -
"Best to view the Ellenbecker video as I could misinterpret. At minute 8 he describes the shoulder orientation to minimize the risk of impingement.
Think of the line between the two shoulders and the line of the upper arm. If those lines fell in a straight line, the angle is 180°, and the shoulder internally rotates (upper arm axial rotation) well and impingement risk is low. As the upper arm rises relative to the shoulders the risk increases. I have read that 170° is still OK. Let's say that it is higher than recommended, at 150° for example - then there is more risk of impingement.
You can also visualize the angle by extending the shoulder-shoulder line and considering the complementary angle. Then for example, 0° is upper arm straight out, 10° is the upper arm up slightly and still OK, 30° is the upper arm up and at increased risk of impingement.
I see many players serving well beyond the 10°.
The shoulder orientation requires lateral trunk flexion which may stress the back for some people."