Hi Martin,
is actually not a big deal. However, I can't describe it very well with words

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There's a kind of mechanism there that works like a cogwheel railway. You also have to adjust the belt tension after working with it. But first, you need to loosen some screws on the power steering pump so you can move it... Take a good look at it; it's actually quite self-explanatory. The pulley can definitely stay in place.