The action or flex of the fly rod impacts on how the rod will cast over a range of distances, how it will perform fighting fish, how it will protect tippet from breaking and what type of flies it will present the best. In general terms, fly rods come in fast, medium fast, medium and slow actions. The action of the rod is a direct reflection of how deep the rod flexes into the blank while casting. Having designed many rod tapers, I can guide you the right rod taper design for your needs.
A slow action or full flex rod bends deeply into the blank, in some tapers into the butt of the rod. Slow action fly rods have a very soft feel. They excel in lighter weights and shorter lengths for small stream dry fly fishing for trout since they load very easily and are capable of soft presentations at short ranges. These rods also protect light tippets from breaking very well since the full flex action acts as a very effective shock absorber.
Medium action fly rods are a good choice for an all-purpose freshwater fly rod for everything from trout to larger fish like steelhead or northern pike. Medium action rods flex into the middle of the rod and load easily but have the lower blank strength and stiffness to punch out longer casts and fight powerful fish but have enough flexibility in the upper half of the rod to make accurate and quick short range roll casts. Medium action rods are also capable of making good presentations with dry flies, nymphs or streamers.
A moderate fast action is a great mix of versatility and performance, providing a deeper flex into the blank then a fast action rod. This action allows the angler to comfortably switch their fishing technique depending on the fishing situation for instance, nymphing to dry flies. Performance wise these rods will still punch tight loops in the wind, while offering a level of flex to protect you from breaking off that fish.
Fast action rods are fairly stiff and flex only in the upper third of the rod. They excel at long casts with heavy flies in windy conditions. Fast action rods require a longer portion of fly line to be in the air to load the rod and cast well. Fast actions are also a good choice for western float fishermen who drift big rivers and enjoy firing big hopper patterns or streamers inches from the bank where big brown trout are lying in ambush.
Progressive action rods excel at using the entire length of the rod as a lever to cast long heavy lines. As the rod loading is increased the rod bends further and ("progresses") into the butt section. This action is perfect for spey style rods.