Check out the novice dowser in the picture above. He is not using a traditional forked stick divining rod (see article below). He’s holding a couple of welding rods, with a 90-degree bend, forming an L-shape. The shorter ends of the rods are held in upright fists, with the longer ends pointing out straight ahead, horizontally and parallel to the ground.
The two rods must be gripped hard enough to keep the long ends horizontal, but not so hard that an imperceptible inward motion of the wrists won’t allow the rods to cross in front of the dowser. Using this method, when the dowser gets over a "vein" of water or a hidden item, for example, the crossing of the rods indicates "pay dirt."
Another device used by old-timers was the pendulum. A simple plumb bob will suffice for this purpose. Chester Clift, who was a well-known dowser from Washington County in New York State, favored this method. (Chester used to spend the long upstate winter fashioning pendulums out of unusual rocks and stones for his many friends and followers.) The motion of the pendulum, held on a short string by the dowser, naturally follows either a clockwise, counterclockwise, or a back and forth motion. With this method, the individual dowser decides which of these motions indicates the presence of water or other objects and interprets the results accordingly. And of course, as you'll see if you try it, you can control the motion of the pendulum with the slightest movement of your hand.
In all methods of dowsing, whether with a forked stick, metal rods, or a pendulum, my conclusion is that the operator already has an "educated guess" as to where to find underground water, or even lost objects. There’s no magic to it, other than "divining" what the dowser already knows. Anything more than that is the realm of ignorance and superstition, and except for occasionally relating a few quaint stories from my youth, I basically forgot about the subject.