When we talk about rod actions, we typically think of three descriptors: fast, medium, & slow. Sometimes we mix these terms to 'medium - fast', or 'medium - slow'. Most of the time, these are the terms used to describe a fly rod's action.
This seems simple enough, but these terms are vague and not nearly detailed enough to describe the nuances of fly rod actions. Just because a rod is described as, say, medium / fast that doesn't really tell us what we need to know about how it really feels in the hand. The best that term can do is to give us a general idea of how the rod might feel when we wiggle it.
Here I want to discuss two other descriptors that go beyond the typical fast, medium, & slow terms. These additional terms will tell you more about how the rod bends in very specific ways. For those of you new to bamboo you might have heard these terms tossed about before, but weren't sure what they meant. Hopefully what follows will clear things up for you. Please keep in mind that there's a lot more to the terms we're going to discuss, but today we're just going to cover the basics.
parabolic action:
Rods with parabolic action have unique characteristics which you can feel the moment you wiggle one. They tend to have most of the flexing in, or close to, the handle of the rod - down the butt section in the lower third of the rod. In some cases this gives an almost hinge-like effect. Why would you want this? Most parabolic rods are powerful casting tools, allowing you to toss flies into the wind, or allow you to cast a dry fly very accurately at long or short distances. They can pick up more weight for their sizes than other rods (think big, wet flies) while also delivering dry flies gently onto the water. When fishing from a boat, tossing dry flies on a small stream, high-stick nymphing, and in many other situations, parabolic rods can be very useful fishing tools.
How they got to be called 'parabolic' is a bit technical. Rod makers & designers often plot the taper of a rod along a graph chart, to give them a visual representation of the rod's stress curve. This allows them to see how the rod will flex & where along it's length it will bend the most. Overall, to simplify, it tells them what kind of action the rod will have. Think back to algebra class for a minute. Remember when you would plot a line & find the 'rise' of that line. That's very similar to what I'm talking about.
Okay, with that said, parabolic rods are called that because when you plot the line of the rod taper out on a chart, or graph, it forms a rough parabola......mostly.......kind of......Rod making isn't always an exact science........but it's always pretty darn close to a parabola! At least this is one popular explanation. The first parabolic rods were designed by trial & error & not by some mathematical equation, so who really knows for sure?
From a fishing standpoint, the thing to keep in mind is that parabolic rods do the vast majority of their flexing (relatively speaking) in the bottom third of the rod.
progressive action:
If you're new to bamboo, progressive action rods are a great place to start. Because of the even flexing of these rods, it's very easy to feel the fly line loading while casting. It's also easier to cast specific distances, too. After al little time spent casting the rod, you'll be able to tell how much force you need to give the cast to reach the spot you want your fly to hit. Even on a windy day, with some practice you can compensate for the wind. Progressive action fly rods become very intuitive casting tools once you get the feel of them & learn their timing.
Fast, Slow, and Everything In Between:
Think of it this way: the speed descriptors tell you how much a rod will flex, while the 'parabolic' & 'progressive' terms tell you about how the rod will flex. This, of course, is an oversimplification but I think you get the picture. Rod action isn't always an easy thing to put into words, and hopefully what I've said here made some sense. If it did, then the next time you hear these terms tossed about by bamboo rod anglers, you'll have a better idea of what they mean.
Lastly I'd like to say that to be absolutely certain of a rod taper, you need to put the rod in your hand & wiggle it. This is all very objective & each of us will have our own personal tastes for what we like in rod actions. The best rod action is the one that feels best to you.