From In Defense of Overhead Lifting–
"A Case for Jerks, Push-Presses, and Overhead Supports
Here’s a tip: there are many lifters who, despite their best efforts, continue to have pain during certain portions of the overhead pressing movement. For many of these lifters however, there’s no pain when the bar is completely overhead in the support position.
For these trainees, variations of the jerk, including push jerks and push-presses, can be valuable alternatives as they help avoid the painful regions of the movement. This is a case where speed or momentum is actually safer than doing the exercise with a slow, concerted effort that’s so commonly recommended by coaches and trainers."
Why is this? Is it an impingement issues or torn rotator cuff issues, etc… and why is the pain less at the top end of the motion.