Parabolic action

[T1A]

I had asked a question and this was the most complete answer I had ever seen.  Thanks to Ouachita  for posting this answer.

A parabolic rod bends more like a parabola (bowl) than a hook. This applies to fiberglass and some composite rods, not generally to graphite.
http://en.wikipedia.org/wiki/Parabola

Action of a rod is mostly determined by the thickness and taper of a rod. It also has a lot to do with ability to release stored energy to aid in distance casting of light lures (responsiveness). When the rod is whipped back, you are bowing it, storing energy in the flexing part (action area). When you let the rod-bow straighten out you are releasing that stored energy. Not all slow to medium slow action rods bend truly parabolically, but are close to true. Some bend progressively, more and more angle as you look from handle to tip. Those rods have progressively less sensitivity towards the tip because stored energy in the rod tip builds up when put under a load. Vibrations coming up the line are not transmitted with equal strength from tip to the end of the rod flex. Some rods like that will let the rod tip flicker very noticeable with a little bream hooked, but no vibration is felt coming through the handle area. The higher the power, though, usually the more "sensitive" or vibration transmittal the rod is. It's actual incorrect to say a rod is "sensitive". It's a good conductor of vibration down to a bad conductor. Conductivity.

Most rods bend little or not at all from handle to about half way to the tip. The bending is most noticeable in the tip 1/4-1/3 of the rod length past the handle for fast action. Moderate action bends about half way between tip and handle. Slow action bends close to the handle throughout the rod length past the handle.

Rod power is how much force it takes to bend the rod. If you can bow a slow rod yourself, it's probably a ultra light to medium light power rod. Its action is determined by the way it bows as described above. If it takes considerable effort to bend the rod into it's rated bow shape, it's probably a medium to medium heavy power rod. It might take two people grunting to bend a heavy to extra heavy rod. The concept of power is similar to an archery bow. They ar rated in terms of pounds of pull.

When selecting a rod keep the lure type, expected obstructions, fish species, and line type and strength in mind. You then must decide which combination of features you need.

I like parabolic curvature in a moderate power, moderate parabolic action (evenly bowed along the length but not as a slow action that can make a half circle when flexed) fiberglass rod for some lures, like crankbaits, that have the same responsiveness and conductivity from tip to handle. Whatever vibration makes it to the tip comes to the handle. Many glass rods don't do that well because they are too cheaply constructed. Graphite rods are much more conductive, but are more brittle than glass rods. Seems odd, huh? Glass? FIBERglass. That's why some of the best rods have those and other materials built into the rod blank (composite).

As a line watcher in worming and jigging, I wouldn't want that there. I would want a highly conductive rod like graphite or a composite graphite+glass+carbon rod that isn't as responsive as glass, but those are very expensive to make (and buy). I prefer reasonable casting distance matching line and rod rating, but mostly the enhanced conductivity of vibration of graphite combined with the power of fiberglass (not liking 100% graphite).

Hope that answers.

Jim