Once the track is off the ground, spin it backward for about 5 seconds to take any slack (due to wear on pins and bushings) from the top to the bottom, so you can get an accurate reading of how loose the track is.
In regular operating conditions, the sprocket pushes the track links toward the front idler, causing the top of the track to compress between the sprocket and the idler. When you lift the track and spin it backward, the sprocket teeth pull the links of the top section of the track, causing the track to stretch between the sprocket and the idler.
You can see the above explanation in the picture below. Pay attention to the black marks between the links as the mechanic stretches the track.