A rod can bend in different curves. Traditionally the bending curve is mainly determined by its tapering. In simplified terms, a fast taper will bend a lot more in the tip area and not much in the butt part, a slow taper will tend to bend too much at the butt and delivers a weak rod. In between is what is called a progressive tapering which loads smooth from top to butt, adding in power the deeper the rod is bent. In practice, the tapers of quality rods often are curved and/or in steps to achieve the right action and bending curve for the type of fishing a rod is built. In today's practice, different fibres with different properties can be used in a single rod. In this practice, there is no straight relationship anymore between the actual tapering and the bending curve.
A fishing rod's main function is to bend and deliver a certain resistance or power: While casting, the rod acts as a catapult: by moving the rod forward, the inertia of the mass of the bait or lure & rod itself, will load (bend) the rod and launch the lure or bait. When a bite is registered and the fisherman strikes, the bending of the rod will dampen the strike to avoid line failure. When fighting a fish, the bending of the rod not only enables the fisherman to keep the line under tension, but the bending of the rod will also keep the fish under a constant pressure which will exhaust the fish and enable the fisherman to actually catch the fish. Also the bending lessens the effect of the leverage by shortening the distance of the lever (the rod). A stiff rod will demand lot's of power of the fisherman, while actually less power is put on the fish. in comparison: a deep bending rod will demand less power from the fisherman, but deliver more fighting power to the fish. In practice, this leverage effect often misleads fisherman. Often it is believed that a hard, stiff rod puts more control and power on the fish to fight, while it is actually the fish who is putting the power on the fisherman. In commercial fishing practice, big and strong fish are often just pulled in on the line itself without much effort, which is possible because the absence of the leverage effect.
Also known as "power value" or "rod weight." Rods may be classified as Ultra-Light, Light, Medium-Light, Medium, Medium-Heavy, Heavy, Ultra-Heavy, or other similar combinations. Power is often an indicator of what types of fishing, species of fish, or size of fish a particular pole may be best used for. Ultra-light rods are suitable for catching small and also , or situations where rod responsiveness is critical. Ultra-Heavy rods are used in deep sea fishing, surf fishing, or for heavy fish by weight. While manufacturers use various designations for a rod's power, there is no fixed standard, hence application of a particular power tag by a manufacturer is somewhat subjective. Any fish can theoretically be caught with any rod, of course, but catching panfish on a heavy rod offers no sport whatsoever, and successfully landing a large fish on an ultralight rod requires supreme rod handling skills at best, and more frequently ends in broken tackle and a lost fish. Rods are best suited to the type of fishing they are intended for.
Back- or butt-rests can also be used with modern fishing rods to make it easier to pull big fish off the water. These are fork-like supports that help keep the rod in position, providing leverage and counteracting tensions caused by a caught fish.