The secret?
There are no secrets. It's not magic. The rods do not "find" water.
And yes, if the water table is 15 feet in my area I will hit water at 15 feet no matter where I dig. But some areas give you more access to that water and that is where dowsing comes in.

My opinion, for what it is worth, is that your own body is somehow sensitive to the voids beneath the soil you walk on. The rods are merely a way that your body shows you what it is sensing beneath your feet. A visible meter, so to speak. And some people are more sensitive to the feeling than others.
These voids are created where water lines are buried, cables are buried, septic lines are buried, where graves exist - and yes, voids where water can more easily pool. So the rods are not "finding" water. It's your own body sensing the void where water is more easily accessible.
Like any other skill dowsing takes practice. I am not a dowsing advocate but I do encourage people to try it. Try to find your buried water lines or cables then explain to me why and how it works. I don't pretend to know but I have used them with a good track record locating all of the examples I've listed so I use them just like any other tool in my toolbox.
I have learned that this is a very divisive subject and my original intent was merely a suggestion for people to try - but that single suggestion seemed to take on a life of it's own with no middle ground.

Give it a try or don't. It really is as simple as that.