I saw my mechanic one time to take the belt in two places and try to force to move from one side to other. he explaines me that the only moment in which you can move it is when the chain is loose and need to be replaced.
He turn car off, dismount the upper side of the radiator fan cover and took belt with both hands, left between air compressor and exhaust compressor and right between alt and hidraulic pump... then tried to make it move in both directions. the belt moved a bit and stopped and the same for the other side. He dissasembled and replace the chain and tooth wheels.