The tool installs in notches in the hinged tensioner brackets; the notches are outlined in the left photo. The thinner bracket on the right is fixed by two bolts. The other bracket can rotate, allowing the tensioner to expand or compress as needed.
When the tensioner pulley bolt is pushed toward the rear of the car, the tensioner is compressed and the lower notch rotates upward. Installing the tool locks the tensioner in the compressed position, allowing removal of the tensioner pulley, and then the belt. Note that when you loosen the bolt - pulling forward on it, as it is left-hand threads - the tool prevents the bracket from rotating, which prevents the tensioner from expanding and separating.