Terminology used on the Stronglifts.com wiki entry on the Overhead Press, consistently refers to it as "Shrug your shoulders to the ceiling." That suggests that it is indeed the shoulders, and the image looks like it takes the shoulder up towards the neck:
Shrug your traps at the top. This creates space between your upper-arm and AC joint for your rotator cuff tissues. No shrugging is no space is shoulder impingement.
Shrug at The Top. Shrug your shoulders towards the ceiling at the top. This engages your trapezius muscles and prevents shoulder injuries. The rep doesn’t count if you didn’t finish it by shrugging your shoulders. Press the bar until your elbows are locked. Then keep pressing by raising your shoulders up. You’ll achieve a stronger lockout position, build bigger traps and avoid shoulder impingement from the Overhead Press.
Based on their video, it looks like the entire shoulder is raised, all at once and it goes all the way up.