You're right it should be at the end of the shaft and not move, but in that photo it looks like it's stuck to the damper end at 30% sag.
It’s just been slid up to mark the 30% sag point for the photo. Even if it was left there it would slide back down with minimal resistance and stay there.
@TotalShanner started
London Fixed Gear and Single-Speed is a community of fixed gear and single-speed cyclists.
You're right it should be at the end of the shaft and not move, but in that photo it looks like it's stuck to the damper end at 30% sag.