First let me say that the dowsing rods are not magic! They are just "L" shaped pieces of wire with no special ability other than to conduct energy. They move because of interactions between your energy field and the energy field of whatever you are looking for. It is all about intention! As you hold the dowsing rods, they will not extend beyond your energy field. So it would be impossible for them to sense another field. What they sense and respond to are changes and fluctuations in your own energy field as it interacts with other energy fields. If you intend to find a "healthy" energy field, then the rods will move whenever your energy field interacts with what you consider to be a "healthy" energy field. By simply changing your intention, you can keep the rods from moving. In high school physics you learn that when ever one electric field crosses another electric field, a magnetic force is generated. It is this force that is actually making the rods move. (It is important you realize that your field must be moving in relation to the other field in order for the resulting magnetic field to move the rods.)
Making your own rods...
Start off with two pieces of stiff, solid, wire. I prefer thick (4 AWG is size) solid copper grounding wire, but thin coat hanger wire will work fine. Copper grounding wire is available at almost all hardware stores. When using thick wire, I compensate for the thickness by starting with two pieces about 22.5 inches long (22 inches for thin wire). At about 6.25 inches (6 inches for thin wire) from one end, place a 90 degree bend in the wire. To ensure a sharp bend, place the wire on the edge of a counter and bend it. This short end will be the end you hold. You want to end up with two pieces of wire that have a 16 inch long section with a 6 inch handle. It may be helpful to place the short end of wire into a tube... that way you can hold the tube and the wire is free to move unencumbered by your hands… a large straw or small plastic pipe is good. Make sure the tube is made of plastic or some other NON-conductive material. I use PVC piping because it is cheap and can be bought by the foot in various sizes. (The tube is particularly helpful if you use uncoated wire like the copper grounding wire as some people's hands react to the copper and prevent the rod from moving freely.)
Using your rods...
To use the rods, loosely hold them (or the tubes they are in) in your hands like you would a knife - but not too tight as they must be able to spin freely. The long ends of the wire should be pointed away from you and just slightly below horizontal. It doesn't matter if you hold your hands close together or far apart. In fact I suggest you experiment. Remember: Intention is everything!!! Begin with something simple like a crystal on the floor. Intend that the rods show you where the crystal is. So set your intention and then walk forward. What happened when you walked over the crystal? Now change your intention for the rods to show you positive crystal energy. What happened? This time intend the rods to show you negative crystal energy. Did you notice any difference? The more specific your intention is, the more specific the response will be.
When I teach this in a class, I play a game with my students. I place about 6 plastic cups on the floor. Underneath one of the cups I place a quartz crystal that I charged up with Reiki energy. I then ask the students to use their dowsing rods to find which cup the crystal is under. When you walk over the cup with the crystal the rods will move by themselves. I've never had a student not be able to find the crystal!