This is a technique that is useful in the studio or workshop setting,   and designed to help the actor overcome moments of difficulty concentrating while doing an exercise or scenework.   It's one of my favorite techniques, because using it helps to identify immediate problems with blocks to concentration. 

Walt and Sue are doing the scene from Tennessee William's "A Streetcar Named Desire" in which Blanche and Mitch meet for the first time.  It is obvious that things are working well for both of them, that the objects of attention on which they have chosen to concentrate are serving them properly.

Sue comes to the line, "Sorrow makes for sincerity, I think", but hesitates.  A flicker of doubt crosses her face.  She then "speaks out" her frustration, "Dammit! I keep hearing in my head Vivien Leigh's voice saying that line... this is so frustrating."   Having identified the "block" (hearing Vivian Leigh's voice in her head) grounds her "in the moment".  She shares with the others in the workshop that she too knows what they perceive:  she is having a "moment of difficulty" at this point in the scene.  Rather than continue on this "false note", she verbalizes the problem.  Then she may take a moment to relax, re-focus her concentration, and when she feels "there", the line "sorrow makes for sincerity, I think. "  now rings true.  Walter, affected by her new found truthfullness, responds with, "It sure brings it out in people. "

And the scene continues.

Everyone in the workshop here knows what Sue was doing when she " spoke out" her thoughts on the stage during the scene.  The voice of Vivien Leigh from the first movie version of the play was ringing in Sue's inner ear, causing a break in her concentration.   She felt she could not say those words truthfully, but would end up imitating Vivien Leigh's unique "Blanche".  Sue doesn't want to imitate another actress.  Sue wants to own the "life" of Blanche.

So Sue employed the technique we know as "Speaking Out" onstage in her "moment of difficulty".  The actor can "Speak Out" any problem relating to his work in the moment.  If the actor feels tension, he can, wherever he is in the scene, remark, "I feel tension. It's blocking me.", stop what he's doing to correct the problem (tension), then continue with the scene.

This technique serves at least two valuable purposes.  It let's the instructor and students know that the actor onstage is aware of a problem, as well as the cause of the problem.  It also serves as a "grounding" mechanism for the actor, who, when "Speaking Out", is automatically saying something that is true and real.  Regaining this sense of truth often acts as a springboard to the next moment of truth, and the next, and so on.

I have discovered since I have been teaching, that occasionally actors become overly dependent on the technique of "Speaking Out", and half the scene becomes the actor's thoughts about what his problems are, rather than addressing the basic problem itself, which is usually a break in concentration.  At this time, I ask the actor to change his object of attention, and try the scene without using the technique at all.