You often hear singers talk of singing naturally, as though singing were an inherent bodily function like vision or walking or blood circulation.
The Alexander Technique community, too, suffers from the widely held belief that good singing is natural, and thus can be elicited in anyone if we can just get him or her free enough. I have heard this implied even by Alexander teachers who are also singers.
The muscles we use for singing evolved as parts of our swallowing and breathing mechanisms. They open the throat to facilitate the passage of food and air and close the throat to prevent things from going down the wrong tubes. It is a marvelous serendipity that humans have figured out how to co-opt these muscles for producing the glorious sounds of singing. In order to properly coordinate the "vocal" muscles (e.g. cricothyroids and arytenoids) for high-level singing, they have to be coerced into interacting in highly unnatural ways that they would never experience in performing their original functions. In simplified terms, muscles groups that normally alternate their action--either these or those contracting at any given time--must learn to contract simultaneously and brace against each other's pull.
To achieve this unnatural but agreeable coordination is perhaps one of the most difficult human pursuits. Cornelius Reid used to assert that only a few singers in the whole world in any era manage to coordinate their "vocal" muscles at the highest level of proficiency. (He meant few enough to count on the fingers of your two hands.)
Faced with the challenge of eliciting such an unlikely coordination, it makes sense for all singers to employ the Alexander Technique to make sure that we aren't creating unnecessary tensions that would make good singing even less likely. But, contrary to widespread opinion, simply freeing yourself of tension won't coordinate your vocal registers. An Alexander lesson in itself will not turn you into a master singer, any more than it will have you dancing like Baryshnikov. (Unless you are already a Baryshnikov. Or a Björling. Or a Beyoncé.) You need voice lessons too, and preferably from someone who understands how to elicit that elusive and unnatural response from the muscles of your larynx.