The tensioner is a spring loaded eccentric held by the center fastener. You’ll have to pry this outward and tighten the nut to hold it while routing the belt. Loosening and tightening the tensioner nut will set it in place.
Volvo 240 Wagon Timing Belt Replacement + Tensioner Tips
Before removing the old belt, loosen the tensioner nut and pull or push on the belt. You’ll see the tensioner move and understand it’s action better. Reloading the spring and tensioner takes a bit of concentration.