人
The kanji for "person" is simply two strokes: the first from the top of the character and sweeping down to the left, and the second from the top of the character and sweeping down to the right.
Using your finger to draw this kanji on the palm of your other hand will relieve tension when faced with a "hostile" crowd of people. Hostile, not necessarily in the sense of aggressive, rather any crowd which makes you feel tense. For example, before giving a sales presentation to important customers. Having drawn the kanji in the same spot three times, raise your palm to your mouth and "drink" those three "people".
One common explanation of why this works is psychological; you have consumed your audience and they are no longer a threat to you.
Another explanation is that when drawing the kanji, you are also stimulating an acupressure point in your palm.
The calming result could be one or other of those two reasons, or a combination of both.
A third explanation is intentional or unintentional autosuggestion.
The drinking activity in this case is gulping air. Do that slowly and yes, just slow breathing (no need to draw the kanji) can ease tension. Remember, however, that repeatedly inhaling great gulps of air could easily lead to hyperventilation, increased heart rate and increased anxiety.
There's also the risk when the activity is done to relieve mild tension becomes habitual. According to the Pavlov conditioning theory, the activity could actually increase tension.
Whatever way you find to become calm without any of the risks mentioned above, will be the best thing for you. And keep in mind that a little bit of tension can be beneficial.