This is the post which stops the ball from making it all the way around the orbit and instead goes into the VUK for the Path of the Dead.
It seems to activate randomly but I'm guessing there is something that triggers it. Also, my post works but often when during gameplay the ball may graze the post or something but the post doesn't slow the ball enough to have it drop into the VUK.
The reason I ask is that oftentimes during a game I find it hard to get the ball up to the POTD and almost always start the fellowship and two towers mb's way earlier and easier than RotK.