It demonstrably doesn't work.

Well, it "works" in the same way that a Ouija board works. There's a person in control of when the rods cross, just like there are human(s) in control of the planchette on a Ouija board. It's very easy for a person to influence the movements of both, without necessarily being conscious that they are doing so.

Inevitably, the person who is holding the rods has some idea of where to find water. Like others have pointed out, there are hints on the surface, like trees and plants, even the shape of the terrain. Once they have an idea of where to look, it's very easy to make the rods cross at that point.

And if there's not water there, then there's always an excuse. "The energy was not good that day." "There's an underground stream, but it's dried up now." "There is water, but you didn't dig deep enough." etc.

And of course, they almost always find *some* water because there's water nearly everywhere if you dig deep enough. And confirmation bias being what it is, they will always point to their successes as evidence that dowsing works, and their failures are just times where they did it wrong.

Again, they may not even realize that they're doing this. They may really believe it's magic.

Some don't of course. Some people are just charlatans. But there are plenty of true believers too.

Whichever they are, they don't have a real ability. Every time we run a properly controlled experiment, they do no better than random chance.

The James Randi Foundation used to do these tests, offering a million dollars to anyone who could demonstrate any one of a number of abilities (dowsing included). No one has ever successfully passed even the preliminary tests.