I don't think I see much
internal shoulder rotation (ISR) leading to impact. Looks like the common error where edge leads and impact occurs with a slightly different angle. See serve at 49 sec. View single frame. Last two look like kicks. Same issue.
To do single frame advance on Youtube use the "
."and "
," keys.
To directly see your ISR before impact, tape a piece of blue painter's tape on your upper arm just above the elbow.
Study what ISR rotation is and what it looks like in videos. Search forum for my posts on internal shoulder rotation.
Serve with ISR. Imagine blue tape on part of arm between elbow and shoulder joint.
To do single frame on Vimeo, click Vimeo, full screen, hold down the SHIFT KEY and use the ARROW KEYS.