Kauyon

I've searched everywhere I can think of but I can't find an answer to this bloody question. Recently I was doing a spot check under the hood of my SC400 checking all the fluids and stuff when I noticed something peculiar, one of the idler pulleys (if it is in fact that) moves sideways while the engines running, it has 4 notches 3 being close together and the other far off to the right, with an arrow pointing towards the notches, while the engine is idling it moves back and forth between the markings. My question, basically, is it supposed to do that? I've literally spent hours trying to find a picture showing it but I can't for the life of me find one. Can anyone shed some light on what this is, and what those notches indicate? I'm guessing its some sort gauge for either tension, wear or bloody hell I don't know what. I attached a pic of the closest I could find to a picture of it.

EDIT: Figures I would find a bloody picture showing it after I make the post. Heres a direct shot of the pulley I'm talking about.

over40driv

That is your automatic belt tensioner and the notches tell you the amount of stretch the belt has. As long as the pointer is between the 3 notches, I think your OK. The tensioner moves up and down with the flex and load of the accesories [alt. A.C. & so on] and RPM.