This has always been a difficult subject to put into words. Graphics even, still are lacking in desription. Two things here. Accelerate
and Straight line path
of rod tip.
Acceleration simply means to increase the speed. But the rate of acceleration is of paramount importantance. The rate of change of velocity with respect to time. It must be a gradual increase
in speed. All the way through the stroke! I am reluctant to use the phrase "constant acceleration" due to this reference of SUAS. As if the stroke is made at a constant speed until the final few inches before we finally initiate this rapid SUAS. This is, IMHO, misleading. This technique, what I refer to as a delayed application of power, will only work in the context of classic old style casting. (single hand rods) Where you were taught to hold a book between your casting arm and your body. No double hauls. And you are only casting small flies for relatively short distances. When you start trying to throw large flies, weighted flies, and/or going for more distance, you simply cannot get the desired results with that delayed and sudden, application of power. After all, just how much is the human body physically capable of accelerating in, say six inches, of hand/wrist movement?
Now for the straight line path of the rod tip. We have all seen demo's where the caster makes the classic 180 degree arc, constant speed resulting in no loop at all, not going anywhere cast. And then we are told to shorten the stroke, making a smaller arc, and viola, (sp) the loop forms nice and tight and "out she goes"
Somewhere along the line we are told about SUAS and "where the rod tip goes, the line follows" Fine, but it all happens so fast that it is hard to see and understand, even when you are seeing it live. What they never seem to get around to is the fact that as load is applied to the rod, it bends. Hell we all knew that. But what is important here is that as it bends, the tip is now at a lower elevation as it swings through the casting arc than if it were a broom stick, with no delection at all.
Graphic illustrations attempt to show this. However, they usually fall short in that they only show the rod straight and unloaded at the very beginning of the stroke. The next pic will show the rod fully loaded. The final pic will show the rod after
the SUAS where it has recoiled and showing the small arc the tip has inscribed between fully loaded and fully unloaded.
Allow me to reiterate here to empahasize a point. What they never seem to get around to is the fact that as load is applied
to the rod, it bends. Conversely, the more load is applied to the rod, the more it bends. The more it bends, guess what? The tip drops in elevation. Now before someone chimes in and says yeah but this can be made up for by,,,,,, hang with me a bit longer.
There is a third element that is involved here. Time!
The rate of change of velocity with respect to time. How much the rod bends at any given time during the stroke. This steady increase in speed, which causes the tip to deflect in such a manner as to inscribe this "straight path" combined with just the right amount of "arc" at the butt of the rod will get you, not exactly that "straight" path of the rod tip but one with a slightly higher elevation somewhere between the start and the finish of the stroke. Hopefully, resulting in that classic tight loop.
And this is what separates the men from the boys. The smooth, steadily increasing application of power, culminating in a sudden stop. Allowing the rod to unload, the formation of that tight loop, to catapult the line into the next county.
Not that I am an expert, far from it. Or that I can do it every time. Just do like I say,. Not as I do.