To be honest I haven't a clue but many actors and singers use it (it's all about muscle relaxation and control I believe)
I mention this as G-Man (from the Half-Life video games) was based on Alexander Technique practitioner Frank Sheldon (mentioned HERE). The 'making of' book of the game shows how the game's creators took video and stills of Frank's face running through cycles of facial expressions to enable the programmers to maniuplate the character's face to recreate moods and expressions as demonstrated here
Learn to develop a mood mentally in your head. Your expressions should then reflect that mood and as a consequence look convincing. A decent photographer should be able to help you develop the required mood.
I have long been giving this advice to models. It is equally as effective as an aid to getting the requested pose 'right.' The inner thoughts translate into body language almost automatically.
How can a shot work if you are sitting on a sharp, hard, pointed rock with the thought going though your head is, "How much longer ? this hurts!" and the photographer calls hor a happy smile ?