This breathing technique aims to bring down the temperature of your body and your mind. The hissing sound is produced when you inhale air through the gaps of your teeth.
Here is the set of instructions:
Begin with an easy seated posture and rest your palms on your knees - Sukhasana ("Easy Pose")
Gently close your eyes and take a moment to focus on your breathing
Bring your upper and lower teeth together, and separate your lips comfortably. (In other words, come to a smile exposing your teeth!)
(Inhale) Allow air to enter through the gaps of your teeth, creating a gentle hissing sound
Gently close your lips
(Exhale) Allow air to escape through your nose slowly
Repeat from Step 3 at your own pace
Through this technique, you will be able to feel a calming effect in your body and mind. It is said that if you practice this regularly, it can reduce your hunger pangs - try it and observe for yourself!