In short, the more tense you are, the faster the beeps become, and the more relaxed, the slower. Using this device you can learn how to regulate your body to speed up and slow down the beeps; hence increasing and decreasing muscle tension. With some practice, you will have enough understanding of your body to be able to control muscle tension without use of the device. This is cool because it allows you to consciously control a part of the body you would not normally be able to otherwise sense or easily control.
I set mine up to monitor the muscles in my shoulder and neck that are responsible for tension headaches, but you can place them on just about any muscle group. I recommend experimenting with the placement of the sensors and seeing what is possible.