In case you haven't heard of it the term "hot spot" in fly tying, it simply means adding brightly colored fluorescent material (beads, thread collars or tails) to your fly. The Hot Spot technique originated in the United Kingdom as a way to interest and excite fish from greater distances. The thought process here is once the fish is interested and swims over they very rarely turn down an easy meal.
As far the science behind hot spotting flies goes, fluorescent colors are able to absorb any color of light in the spectrum and give off their own color. Furthermore no matter what the wavelength of light is that gets bounced off the fluorescent thread or bead; it will reflect the fluorescent color back. This is why the hot spot is so productive, as other colors start to fade or change in the water column; fluorescents stay true.
This concept has been used in other styles of fishing for years (trolling baits finished with UV paints, bass baits molded with UV dyed plastics) so applying this concept to fly fishing was only the next step.
To incorporate a hot spot into your fly you should start by thinking of where the placement would generate the most attention. Beads, collars or tails are the three most standard places to add hot spots to flies, but really any material can be substituted for a fluorescent alternative.
When thinking about hot-spotting my flies, I try and keep in mind that if I am going to be fishing in areas that are heavily pressured then I try and keep the added hot spot as subtle as possible. For example, the flies I use on pressured waters will have hot spots placed in partial tail segments or single segments of the abdomen as compared to unpressured streams where I use hot spots in the beads, thread collars and full bodies. Fluorescent Pinks, Oranges and Greens have been what work best in our area (Northern Wisconsin) but don't be afraid to experiment because every stream and river seems to have its own hot color.
If you have not tried fishing Hot Spots on your favorite flies then this spring should be a good one for you. Hot spots are a simple and effective technique to generate some excitement when the bite slows down by bringing a little color into the water.