Timing Belt Replacement

[Iluvmycar]

Hi all. I have begun the task of replacing the timing belt, water pump et-al, and have been following the guide that was posted and pinned by Kevin Watkins. I have replaced the water pump and I am about to tackle the tensioner assembly and the idler before I fit the new timing belt. At the risk of sounding totally bonkers and should never be let loose on a pushbike never mind a car engine, could someone tell me what the hook type thing is that comes out of the face of the tensioner. I can see from the photos that on Kevins original it is not there, (same as my original). When the new tensioner is fitted ,it is there (the same as the one I am about to fit), but the later photo shows no hook

What purpose does this hook serve? Am I losing the plot?

[Iluvmycar]

Ha !! just spotted that the "key" is removed once the tensioner is in place. Could this be one of my better "homer " moments ? :D