Any change in the tension, increased, or decreased is telling you something. This change in tension can be produced by the dog or the handler. No matter who changes the tension for what ever reason it should be telling the other something. The proper tension on the line will keep it straight in the air from the dogs harness to your hand. This allows me to change the tension on the line from as little as a couple lbs.
Lets start from the middle and work up and down with the amount of tension on the line.
You are on the track, the dog is tracking well, and everything is going just fine. The tension on the line is enough to keep the line straight from the harness to your hand. Any change in that tension and something is being said. Keep in mind that all this communication is in conjunction with you reading the dogs body language. As you track along the dog starts to lift his head and move it from side to side as if looking for the track.