Dowsing has been around very nearly forever, and has been (and still is) practiced by many devoutly religious people. There are some who would claim it is Witchcraft, (and persicute those who practice it) and others who would not. There are still many farmers and other ordinary people who dowse for water, in fact, even today, most well drilling companies can put you in touch with a dowser, if they don't have one on staff.
Dowsing
A Y- or L-shaped twig or rod, called a dowsing rod, divining rod or witching rod is sometimes used during dowsing. Dowsing is a type of divination employed in attempts to locate ground water, buried metals or ores, gemstones, oil, gravesites, and many other objects and materials, as well as so-called currents of earth radiation (Ley lines), without the use of scientific apparatus.
You can find rare candies by smashing rocks anywhere and also by using your dowsing rod(in your key items) and use it 2 find items in caves with roks that AREN'T breakable.
Divining or dowsing rods are used by many people. They are most commonly associated with people dowsing for water, but can be used to find any number of other things including but not exclusively to answers, graves, lost things, and treasure.
Short answer is, they don't. Long answer is they rely on the ideomotor effect. It is so easy to move the rod that even barely noticeable movements will effect the direction it turns. Basically the user of the rod is subconsciously guiding it themselves.
Pokemon Diamond & Pearl: By surfing - Golduck Using Old Rod - Magikarp Using Good Rod - Magikarp and Goldeen Using Super Rod - Gyarados and Seaking Pokemon Platinum: By Surfing - Golduck and Psyduck Using Old, Good, or Super Rod - Magikarp
Using the Old Rod in Pokemon Ruby (or any Pokemon game for that matter) will get you a Magikarp as that is the only Pokemon an Old Rod can catch.
By surfing, diving, fishing using the old rod or good rod, breeding, or trading.
You can catch a magickarp using an old rod in sapphire by simply combine it with AR cheats.
you can catch feebas by using a good rod..however, chances are small compared to super rod
Routes 225, 226, and 228 fishing fith the good rod. I caught a Poliwhirl on route 228 Using the Super Rod. and a poliwirl can be cought using a super rod on route 225
Tom Graves has written: 'Pullers' 'The dowser's workbook' -- subject(s): Dowsing, Divining-rod, Problems, exercises 'How to Break into a Computer Career Through Self-Study' 'Real enterprise-architecture'