The terms fast, moderate (medium), slow, etc are all generally accept by fly anglers. Pretty much everybody has an idea of what is meant by those terms when it comes to fly rod action. But start reading about rod design & manufacturer literature & it won't be long before you encounter more technical jargon. This jargon can be very confusing. Listen, some rod makers & manufacturers throw this jargon about in an attempt to sound knowledgeable. The truth is you don't need to know what is meant by 'rod deflection' or 'elastic deformation' to choose the proper fly rod. What's really important is how the rod feels to you.
So, when you pick up a rod, what's the best way to test the rod's feel before you even cast line with it? Well, it's generally referred to as "the wiggle test". It seems that everybody has their own versions of the wiggle test. Funny as it sounds, if you don't wiggle the rod correctly, you'll either learn nothing about it's feel or worse, get the wrong information from your test. Yes folks, there is a 'best' way to wiggle your fly rod!!! Seriously though, done properly the wiggle test will tell you a lot about the rod's balance, it's action, weight, & power. So, you ask, just how should one wiggle one's rod?
The wiggle test is best done with both hands firmly on the rod grip, the rod being held horizontally. The rod butt should be pressed against your stomach. Now establish a slow, side-to-side motion with the rod. Go very slow & pick up only a little speed until the rod is moving smoothly with only the slightest efforts by you to maintain this movement. When this is established, you can watch the rod bending & flexing. See how far down the rod from the tip that the curve is beginning. You now know how quick, or slow, a rod you have. You also know how easy it is to work the rod & you have an idea of how balanced the rod is - because its weight is in motion. We're all familiar with the chart below showing rod action in relation to how it bends (please excuse my crude drawing). Well, this chart that you see in all the catalogs is meant to correspond with this wiggle test - performed just as described above.
Through the years I've seen all kinds of crazy ways to wiggle & test a fly rod. I've seen folks bend a rod against the floor - as if fishing upside down. I'm not sure what that told them, other than the fact that the floor is stiffer than their fly rod. I've seen anglers hold a rod out at arms length & spin the tip in a circle - not unlike Zorro with his sword (in fact, just like Zorro). I've even seen someone bend the rod into a "U" shape - one hand on the grip & another on the tip - & then let go of the tip to watch the rod "spring" back to straight. Well, at least now they know how far it will bend before it breaks, I guess.
You don't need to do any of that crazy stuff to test a rod inside. Just quietly preform the wiggle test as described here & you'll quickly know a lot about the rod you're holding.