Pressing the rectangular button turns the unit on. Once turned on, there is always a tone present, even if no buttons are being pressed. The buttons just serve to change the frequency or "pulse" the sound-level intensity. See the User Manual for a more detailed explanation.
When the rectangular button is pressed and held down, the selected frequency pulses on and off every 200 milliseconds. This is useful in a variety of ways. If you are testing a subject with Tinnitus (ringing in the ears) the pulsing option can help overcome this. The "pulsing" can also be used for clearer communication with the test subject. For instance, if you are testing a subject, you can present the "pulsing tone" and ask what they hear. If they don't respond that they hear pulsing you know they aren't hearing the tone either. This helps to eliminate "false positive" results.
Every unit is calibrated on precision equipment and comes with a calibration certificate. The acoustic calibration criteria are traceable to the National Institute of Standards and Technology (NIST) for sound intensity.