As soon as the sudden stop is made at the end of a fly cast some slack is created in the line and the direction of the cast is determined. What is the rod's "tracking," and can it be measured?

I assume by tracking we mean the path along which the rod moves during the cast. The first thing you have to consider is that any twisting of the rod around its own axis will not affect "tracking" so any issue regarding the spine stuff is actually a non-issue. A truly, badly warped or curved rod could possibly introduce a variable where the rod tip does not travel along a straight line, but it would take a very severely warped rod for that to happen. A minor curve won't matter.

The rod will bend in the direction towards the load being applied. So for the most part, it's the angler, not the rod that determines the direction of the cast.

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"Tracking" is a term being used more and more by rod companies to make you believe that their rod moves perfectly in line with the caster's motion. I think its a farce as they all do that. the idea that you can move your arm in one direction but the rod will "track" in another is @#$%&. "Smooth tracking" or "the rod tracks perfectly" are marketing jargon IMO.

Just where in the cast are you experiencing "slack"? Are you casting a fixed amount of line, shooting line or what? I don't quite understand your question.

If you are casting properly there is no slack in the line until you introduce it as you are laying the line down on the water in order to affect the float time of the fly.
ie. when casting down stream just before fly contacts water i will give a side to side flick to the rod to put a wave i the line for a longer float.

Randolph: When a fly caster makes that quick stop at the end of the forward cast the line is no longer being pulled, and its direction has been determined. The slack is introduced into the cast with the line that you "shoot" into the cast after that quick stop. I can't understand how something called "tracking" can change the direction of a cast after that quick stop is made.

It can't really, except that fly lines have "bulk" to them and if you move the rod enough to redirect the line from the rod tip, that redirect will then travel down the line and eventually make it all the way to the tip end. But again, this requires another movement on the part of the angler. The rod continues to "track" along whatever path the angler decides. The angler can't do one thing and the rod do another.

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I sure wish I could build a rod that would fix the occasional errant cast, but the lure goes where my cast directs it. If you felt frisky, you could set up a few cameras and do some video analysis to determine how the rod tracks. However, this would be more useful in helping to coach the angler on how to correct the mechanics of the cast than rod tracking. The input from the angler and the inertia of the line are going to determine how the rod flexes and along which path the tip tracks during the cast.

Although I'm not a fly fisherman, I'm sure it's no different than using casting or spinning rods. If you apply some drag to the line by thumbing the spool (casting), feathering the line (spinning), or slowing the line being shot on a fly cast and redirect the rod tip that you can make small corrections to a cast in flight.

The only thing that happens when that slack is introduced is that the rod has unloaded and will oscillate until it damps its vibrations. During this time there will be a little added drag on the line, but it's not going to change the direction of the cast, rather it just removes energy from the cast shortening the distance slightly. This is one area where keeping a build as light as reasonably possible can help as the rod will stop oscillating more quickly.

I have to agree that 'tracking' is nothing more than marketing jargon. Until there is white paper to go along with it that explains how the tracking is calculated and measured, provides and analysis with standard deviations, then it's just marketing. It's no different than a blank maker saying that our process produces a 20% lighter blank. 20% lighter than what? They never seem to be 20% lighter than blanks produced with prepress with similar properties (for good reason).

As I learned when I started building, a lighter more efficient rod is certainly beneficial to my casting in many ways, but the blank doesn't correct my errant casts.

Very well said.

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