How the...
 

A friend brought over his SUB bass for a setup. What is with that truss rod adjuster?What am I even supposed to do?

Righty tighty...lefty loosey..

Do you mean the wheel shape? Like this one? http://www.thegearpage.net/board/sho...d.php?t=897720

You use a straight rod (my EBMM and SBMM basses came with one) that will fit in the hole, an allen wrench, or whatever in the holes and turn right or left.

Just be careful not to scratch anything up and if you go too far through the hole, the rod will drag on the wood underneath the wheel.

I actually think this is a much better system than "regular" allen wrench adjustments.

All the best,

Bob

+1!