It is one of the principles of Neurolinguistic Programming (NLP). The stance one takes is determinant in how we feel. I reviewed a few weeks ago the DVD movie Against All Odds.
In one scene, James Woods (Jake) tells Jeff Bridges (Terry) something like this: «You look frustrated !...», and Bridges answers: «I'm frustrated because I feel frustrated.»
Bridges feeling had invaded his body stance. I don't know if he really wanted to look like that or just did not want to fight this feeling letting it control his body.
As the above figure, Peanuts, if he wants to feel depressed, the worst thing he can do is stand erect and raise his head, because he'll improve. Keep on whether he is depressed to lower their shoulders and stare at the floor.
So when you feel bad, frustrated, disappointed, etc.., first to do is deliberately change posture. Raise your head, do not drop your shoulders, walk right and don't look at the floor. You'll see that you feel better.
However you'll feel weird, because the state of mind seems incompatible with an attitude, it all depends on what you want anyway. Continue to feel well... or better.
It is your choice...