The latest book on this and related topics is “שאלת המטוטלת”. I haven’t completed all 500 (or so) pages yet, but I will record my first impressions, anyway. He doesn’t just discuss the dowser, in its many applications, but also “muscle testing”, and various forms of energy healing, with and without objects, including Reiki.
The author’s approach, as he honestly admits in the introduction (not unlike Rabbi Yaacov Hillel), is to “explain” why the “gedolim” have forbidden almost the whole realm. In other words, he doesn’t approach the topic disinterestedly in a search for the truth, but as a partisan of those Torah greats who decided to be machmir, just in case (כחא דהתירא עדיף). He may be right and he may be wrong, but as far as I can see, he doesn’t try or manage to prove his point. That’s from the Halachic angle. Nor does he conduct very serious research from the realia side of the equation, to put it mildly.
Not to Pasken, of course, but my own basic assumption on all these questions, contrary to others, is that everything is permissible unless it falls under some problematic rubric.
For example, he quotes the Tanach on “asking sticks”, and makes a convincing case the sticks referred to are dowsers: dowsers are thousands of years ancient in many cultures, and biblical divining rods lasted for a while, so it makes more sense to correlate and assume they are all referring to the same item than otherwise. (This is a lot like the claim we have identified the correct Techeiles, instead of assuming there are two sources for royal blue, one real and the other fake, as parodied hilariously here.)
But don’t you have to do something forbidden first, like mention an impure name, or burn incense, or attribute powers to a competing being? The pesukim mention “asking” (עמי בעצו ישאל), so maybe you have to ask.
In my humble opinion, unless you say or write something, such as in Reiki, or bow, or thank, or something similar, nothing can possibly be assur. If you divorce exercise from its disgusting “yoga” origins, and mindfulness is not called: “__” (whatever they call it in the Indian tongue), or heal people using “energy” (not chi, prana, etc., so as not to mention idolatry or be מייקר שם המינים), how can anything be forbidden?!
And the fact Avoda Zara was formerly given credit for certain effects is irrelevant. Does anyone doubt electricity would also have been attributed to Avoda Zara if it had been known\sytemized by idolaters? Electricity isn’t well-understood, either, so why is the one “natural” and the other “supernatural”? Defining “science” is notoriously difficult.
Again, I’m not ruling Halacha here.
Update: Here is a halachic book on this after my own heart.