It's a technique from Reichian therapy. You don't need to study the system or buy into its assumptions, though. Just do the exercise and feel what happens.
PART 1: Make a facial expression and hold it for a few seconds. Then Relax all of your facial muscles completely. Then make another expression and hold it for a few seconds. Then relax your facial muscles completely.
Do this for about 5 minutes, or until you've made as many expressions as you want to. Make them extreme, subtle, in between... whatever. If you continue doing this regularly, you'll gain increased control over your muscles and be able to make expressions which you can't now (i.e. tensing one particular muscle which you can only tense in combination with others at the moment).
Part 2: Now squidge your face up tight. Tense every muscle. Pull (with your muscles, not your hands) your cheeks to your eyes. Keep your eyes so tight shut you start seeing white. Now start moving the muscles around while keeping your face very tense. You can do things like move your cheeks from side to side, drop your jaw down and move it from side to side etc. Finally, tense every facial muscle very, very tight and hold... hold... hold...
THEN RELEASE
and feel all the tension leave your face. Feel how your face has transformed.
If you do this twice a day, it will transform how you relate to the world through your facial expressions. You will gain greater empathy, be a little more likeable, and be able to release stress and tension from your face at will.
The above technique is best practiced in combination with two others:
1. Pull your shoulders up to your ears. Pull tham so high and tight that it strains and hurts. Be like a turtle trying to draw itself back into its shell. Feel the tension and heat build... hold it... hold it until it really strains...
NOW RELEASE
and let your shoulders drop down as far as possible. Relax completely.
(great for reducing stress and improving posture)
2. Look into the mirror. Try to express strong emotions using only your eyes. See how the muscles around your eyes move as you do this. Make as many eye-expressions as you can.
(this complements the face exercises very well)
If you want to hear it from an expert, or learn more about the system, I recommend Jack Willis:
http://www.reichiantherapy.net/