The action of a fly rod basically refers to how it flexes. This flex should always be taken into account because it has a big impact on the rod's overall casting performance. An excerpt from a prior piece I wrote briefly touches on the various rod actions:
"Actions are often classified as slow, medium, medium-fast, fast, and extra-fast. A slow-action rod is quite soft while casting, flexing deeply down towards the handle and offers finesse but not much raw casting power. On the other end of the spectrum, an extra-fast rod focuses more of its flex towards the tip for less delicacy but much more power and distance capability."
"Extra-Fast" or "Ultra-Fast" ratings (same thing) sit at the uppermost realm of fly rod action designations. A fly rod like this will focus much of its flex very close to the tip, which in turn gives the rod more available stiffness (power) throughout the blank. Since much of the blank doesn't flex as readily as one with a softer action, it's able to handle heavier loads like big flies and/or heavier lines with more ease.
Some anglers use Extra-Fast rods all the time, while others reserve them just for the most demanding conditions. This all depends on your skill, comfort level, fishing conditions, and what line is chosen.
A rod like this is often considered a "specialized" fly rod because this action performs best with an experienced caster that has good timing and a quick casting stroke. Because the blank doesn't readily flex as deeply, it recovers—or snaps back to a straight position—much faster than rods with slower actions. This means that on each end of the cast, the rod is going to unload quickly which requires better timing to initiate the next stroke at just the right time. While a more relaxed stroke is fine for a medium action rod that readily flexes deeper and recovers slower, a quick, precise casting stroke taps into an extra-fast rod better. This stroke helps quickly load the rod and allows the blank to really "pop" the line out with a very tight loop and above-average line speed that can slice through wind with authority.
I’ve fished and casted extra-fast rods that felt quite “numb” at shorter distances. With a short amount of line outside of the guides, you're working with less available weight which can make a particular extra-fast rod lose feel at shorter ranges in comparison to a rod that’s softer. A rod that flexes more readily in the tip and overall will load easier at closer distances. It's also easier to be more delicate with a softer rod as you can use a more relaxed casting stroke and a line with a less-aggressive weight and taper.
With an extra-fast rod, one way to improve short-range distance is by "up-lining."
One of my older posts covers that in detail right here. If you don't want to up-line, it's beneficial to choose a line that's made with extra-fast rods in mind. You'll find that manufacturers will often make lines that are advertised as being made one half or one full line size heavier than the AFFTA standards. Those lines are made slightly heavier to better load these faster modern rods.
On the flip side, an extra-fast rod is capable of casting very far. While false casting a lot of line (or weight) on a softer rod might overtax the blank by bending it too deeply to allow for any additional distance, the extra-fast rod will have a lot more "reserve power" leftover to tap into since it won't flex so deeply when under the same workload.
As someone who naturally has a quicker, more nervous casting stroke and fishes mostly streamers, I've really come to appreciate extra-fast rods. When dealing with big flies, wind, heavy lines, extreme distances, or a combination of all four, these rods can be a real asset in providing that extra needed punch. Even when conditions aren't so demanding, I still really enjoy the crisp, precise, powerful feel that only these rods can provide. It may not be the ideal action for every angler in every scenario, but I personally wouldn't be without one or two on my rack at all times.