Is it a tightening lever, or a locking lever for after having tightened the belt? Mr. HandyAndy - Really
It does appear that it is a tensioner, as in the belts are a set length and releasing the lever gives enough slack to remove the belt to change it and then the lever tightens the belt to the correct tension, there is a belt alinement knob on the side, is this set correctly. Have you got the wrong belts???.