Hi Farastein,
Here is how I do to tighten or loose the belt/chain.
First, I unscrew this bolt so that approximately half of it is visble:
Then I try to “gently” hammer it on it side, protecting its thread by putting a small piece of wood between the head of the hammer and the bolt.
That way I can move it it any direction to either tighten or loose the transmissiom belt.
When finished just tighten the bolt as hard as you can. You need a hex key that is long enough or that has a handle so that you can apply enough torque.
Hope I was clear enough.