I've heard of a compression arm. and i do think that it is the control arm that it's AKA.
Even though this was on the front, this was mine when i had to change it.



The allen key i used with a deep well with an extension resting on it for a brace so that i could tighten up the rear nut of the sway bar link.
this is the new one [without DT's 'digital enhancements']

Like DT told me when i got the new one, there is a nut that you can't see. To gain access to it, simply pull the boot down just a little like you would when you pull your sleeve up on your shirt.