We know the poker face as the blank face. It's supposed to be the most neutral and unassuming position of our facial features.
But despite its pretense at neutrality - it's not always so. When it's unintended - it can be boredom, a process of deep thoughts or evaluation\decision time, in some cases it can even be viewed as hostility.
Why hostility? Because the lack of expressed emotions can be interpreted as lack of cooperation. When you smile at someone and he or she doesn't smile back - you won't see this as a good or neutral sign, it's the same as not responding to a greeting, it shows lack of rapport.
But a poker face can be handy, and not solely in the game of Poker. In fact, it's one of the proven ways we have to regulate our emotions to certain events.
It's called expressive suppression, one of the psychological techniques proven to help our emotional system deal with negative images. It involves keeping a straight face when encountering with threatening, stressful events. The theory is that there's a tight link between how you feel and the way you express it, in this case it's reverse engineering, we suppress emotions by not allowing them to show.
By the way, the other technique is called cognitive reappraisal. This is sort of the opposite route, you change the way you see something in order to feel better about it. In this method you learn to describe the event to yourself in a more positive or neutral light, not by changing what you see - but adding background details to convince yourself to see it from another angle.
Emotional Regulation
First of all, regulating emotions is often a sound idea - and you probably do it anyway unconsciously - sometimes you want to hide anger and to keep your cool, in other times you want to increase it because you believe it can serve you better, for example when you want to reprimand someone.
By consciously regulating your emotions however, you have more control and awareness to how you feel - and therefore act.
I showed you 2 ways to do it, and empirically the reappraisal technique works better than suppression. Why? because apparently if you choose to treat a certain issue before you actually encounter it - helps you lower the amount of negative emotions you may feel afterwards. Suppression on the other hand, works once you already in the process of the feeling - you're already angry/sad/disgusted - you just don't show it out.
So most of the finding today on emotional regulation show that both technique work on some level, but reappraisal has better psychological effect.
It doesn't mean that adopting a poker face is useless, since It works on the principal that your brain and body are a 2 ways feedback system, if you act habitually more calm and neutral - you'll feel more calm and neutral. But if you try to ignore or suppress ongoing emotions you can cause strain - and in prolonged terms, even health problems related with stress.
Long story short - Acting "zen" like may sounds cool and wise, but in order to work it requires a good understanding of your emotional and psychological position. So if that's important to you, deal with it on both fronts:
I. If something really bothers you, and there's not much you can do about it now - approach it from a different angle: Often an objective, "cold", "looking to the facts" attitude can help you focus and relieve you of the emotional tension associated with the situation.
II. Learn positive and calm body language. If you habitually adopt powerful, open, comfortable and steady position with your body, you easily adopt a more calm and focused mentality. It doesn't work only in one direction - "if I'm calm I'll act calm" - but also the other way around.