the belt tensioner is the only one that moves, the wheel is fixed to a alloy arm that has a 3/8 square in it, your 3/8 ratchet slot into the arm which is spring loaded,
by turning it one way it releases the belt tension,you have to hold it there while you remove the belt off any roller, letting go of it causes it to spring back
its spring return so the tension is self setting
refitting is reverse, put the belt on all the other rollers, then rotate the last one the tensioner, hold it there and slip the belt back on
look at the video above at 3:04, he puts the 3/8 ratchet into the tensioner and releases the tension.
its also worth taking a few pics with your phone of where the belt runs, as it quickly forgotten once removed
cheers
Joe