Action of a Rod can best be described as that portion of a rod blank that bends during a cast. In an extra-fast blank, the blank will bend the most in the top 25% of the blank.. In the case of a fast action blank, the blank will bend most in the top 33% of the blank.If you are casting a moderate action blank you would find that the blank will flex in the top 50% of the blank. While a slow action rod would reveal a continuous curve along the entire rod blank.

     So much for the short version on Rod Action. There is much more here than meets the eye as we will soon reveal.

     However let us first try to describe Rod Power. You will see clearly the difference between ACTION and POWER.

     Rod blank power is simply a rating. Based upon the design characteristics of a particular blank model.
Power catagories are as follows:

• UL- Ultra-Light
• Light
• ML-Med/Light
• Medium
• Medium Heavy
• Heavy
• Extra Heavy
• Ultra Heavy

     Now, power simply describes the overall stiffness of a blank, and the blanks ability to React. The reason for power rating is because there are big and small fishey's. You would not use an Ultra-Light to flip into heavy cover for a 10lb bass. Conversely, you would not use a Mag Heavy blank designed for Muskie fishing to reel in Crappie. Oh you could but, man what overkill. That is the basis behind the Rod-Power ratings.

     So let's get back to Rod-Action. Rod action is more closely related to fishing technique. Remember the terms, Extra-fast, Fast, Moderate and Slow? These terms relate to the amount of time required for a blank to reach maximum load, Or in plain english, the amount of time it takes a blank to go from a fully loaded (bowed) position to being straight again.(this last sentence, excerpt from a post by Craig Baugher)

     A blanks action is created by the design of the taper of the graphite sheets! Huh? As of my writing this, blank manufacturers use both tapers that are designed into the mandrels around which the blank materials is wrapped, and taper cut material which is then wrapped around the mandrels.

     Wait? they use multiple taper mandrels and taper cut material. This is how they can design so many different model of blanks. Simply by changing where on the mandrel the material is rolled, they can change the blanks action. Also by changing the taper of the cut material, can the action of a blank change.

     Basically - a slow or moderate action blank will have the material cut in a straight taper.

     While a fast or extra-fast action would have a multiple of compound tapers with less material at the tip of the blank. How fast will the action be? Will depend on where the changes in the tapers occur. Remember that a fast action blank bends most in the upper 33% of the blank.

     So now we have the beginnings of a vague idea on how action not only works but is created in a blank.

     What about the terms HMG, IM, and HS. These terms refer to the graphite material used in the blanks.

HMG= High Modulus Graphite is used to create today's extremely sensitive blanks. It is more densely woven, and therefore requires less material. However, since less material is used the blank walls are thinner. Hence the blanks are somewhat more fragile.

IM= Intermediate Modulus Graphite- the mainstay of most blanks- IM can range from 36 million modulus all the way up to 54 million modulus. Modulus refers to the elasticity of the fibers. The higher the modulus the less elastic it is.

HS= High Strengh Graphite. This fiber is used in conjunction with the other two; to add strength to the blanks.

    All blanks today use a combination of these three fabrics. HS can be rolled on the mandrel in a spiral or at a right angle to the main sheets of material. This creates a hoop effect, to help keep the blank from shattering or the blank walls from collapsing.

To which Craig Baugher added:
But please understand that, what we refer to as graphite, has two measures: Modulus -stiffness/elasticity, and Strain Rate -Tensile/Strength. Know that as modulus goes up, strain rate decreases. IM materials 45 million or less has a high enough strain rate that you need not worry. However, any materials with a modulus higher than 48-million requires something to boost the strain rate. There are numerous ways in which the strain rate can be boosted, but all add a great deal of cost (mostly in R&D) to the rod. The reason high end rods cost so much. So great caution needs to be employed when buying a High Modulus Rod. The words HIGH STRAIN needs to also be present. If it is not, you could have a VERY SENSITIVE, VERY BRITTLE rod. Stiffness without strength IS NOT a very good thing. The reason why people who buy cheap High Modulus Rods buy more rods more often, or get frustrated and go back to cheaper multi-blend rods and bad mouth high modulus rods.

     I realize that I have strayed from the topic of Action vs Power, but these are not simplistic subjects, and deserve all the attention I can muster. As you can see "many" variables go into the design and makeup of a rod blank. Yet, I must also note that this is not where design ends.

     A blank "action" can be altered by several methods which can be employed by the rod builder. First, the builder can alter the action by minutely cutting the tip section. However, this technique is seldom employed.

     A rod builder can also change the action by the type of guides used and where they are placed on the blank. That's right, the builder can actually change the action of a blank, either to soften or harden it by the type of guides used; how many are used, and where they are placed.

     Well that's it in a nutshell. I hope that this will help answer some questions regarding this subject, And that maybe if someone asks you why a rod behaves as it does, you will be able to help them understand. I also hope that this helps to explain a little, the difference between a custom rod and a manufacturered one. Only custom builders seek to design and construct the ultimate fishing rods, Manufacturers seek to maintain the status quo. In otherwords, they strive to make each rod in a model series as identical as possible.

     But as we have learned, there are too many variables for this to be a reality. First, human hands cut the graphite, pre-peg as it's called, and roll it onto the mandels. Everywhere in the process, humans are involved. Thus, no two blanks could ever be the same or fish the same. Even with all of the engineering that goes on to design a blank, ultimately people have way to to much to do with the actual construction of the blank itself.