Whilst the test curve of a rod describes the amount of weight required to bend the tip of a rod round to 90 degrees of the butt, the action or taper of the rod describes how the rod actually bends through its length.
You might be forgiven for thinking that all rods simply bend equally throughout their entire length but there’s actually a lot more to it, and if anything, you should probably pay more attention to the action of your rod than its test curve in order that you select the right tool for the job.
In terms of action; a slow action rod will flex throughout the full length of the blank, whilst a medium action rod is designed to flex more in the top half and less in the bottom half, thus the bottom half stays stiffer. A fast action rod moves the flex still further up the blank so that it is only the top quarter of the rod that will flex, leaving the majority of the rod stiffer.
The taper of a rod describes how the thickness of its diameter changes throughout the length of the blank. A progressive tapered rod will have an even increase in thickness all the along its length and will generally stiffen towards the butt, whereas the diameter of a compound tapered rod may vary along its length in order to manipulate its action.
The mix of the two means that a compound tapered fast/medium action rod will allow you to cast greater distances, as the bulk of the rod stays stiffer allowing the power of the rod to unload better on the cast, whereas a progressive / all through rod is better applied to short and medium distance work where you need to use all the power of the rod when playing big fish close in.
Twin the action and taper of your potential rod to its test curve and you can easily match a rod to your intended requirements, ensuring your rod will not only be up to the job, but will also be fun to use in the process!