Pressing the rectangular button turns the unit on. Once turned on, there is always a tone present, even if the button is not being pressed. The button just serves to change the frequency or initiate pulsing. The default sound intensity level is 25 db HL. See the User Manual for a more detailed explanation.
When the rectangular button is pressed and held down, the sound level changes to 40 db HL and the selected frequency pulses on and off every 200 milliseconds. When the button is released the pulsing stops and the unit automatically reverts to the 25 db HL sound level. 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.