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Spine is a measurement of the amount a shaft will flex when a force is applied to the nock by the loosing of the bow.

Given a right handed archer shooting conventionally (arrow on the left of the bow and fingers on the right of the string, a straight line of force through the shot and a clean loose) a matched shaft should leave the bow cleanly and quietly, recover from flexing and in lateral terms (left/right) go exactly where pointed.

When a shaft is drawn back and loosed, a given force applied by the string pushes the shaft forward.
The inertia of the shaft and the weight any point on the end of the shaft resists this force and so the shaft will tend to flex.
Since the shaft rests against the bow, slightly offline from a centre line from the string through the bow, the shaft first flexes into the bow, then flexes around the bow as it passes and again into the bow as the shaft clears the bow.

When this sequence of flexing is in harmony with the bow (it's power, length of stroke and width of the bow) the arrow will leave the bow cleanly and quietly, and recover to go in lateral terms where the shaft has been pointed at full draw.

This is known as the archer's paradox, but the traditional explanation of paradox is based upon an incorrect understanding both of what occurs, and of what is desirable.

The term "archer's paradox" was coined before arrow flight was fully understood scientifically, and the traditional explanation erroneously describes the shaft as going round the bow, straightening up and going where the bow was pointed.
This explanation is potentially misleading.

What is desirable is that the shaft go in lateral terms exactly where the shaft has been pointed at full draw.
The bow is going nowhere and in any case is in fact pointed slightly offline from the line through the shaft to the target at full draw.

NB. This assumes a traditional bow which is not centreshot and has side contact of the shaft on the bow.

A weak shaft (underspined in American terminology) will flex too much, may strike the bow as it leaves, will shoot somewhat to the right of the full draw line of aim and is likely to fishtail (wiggle left/right).

A stiff shaft (overspined in American terminology) will shoot more stiffly off to the left of the aimed line, but a slightly stiff shaft is generally preferable for practical reasons to a weak shaft.

Factors affecting functional spine are primarily shaft length and point weight.
Width of the bow at the arrow pass and shaft diameter are also factors that have an effect on spine selection.

A longer shaft of a given material and diameter is by definition more flexible.
A shorter shaft is effectively less flexible.

A heavier point weight makes a shaft more flexible.
A lighter point is effectively less flexible.

A wider pass needs the shaft to flex more, a narrower pass requires the shaft to flex less.

A larger diameter of a given shaft material is in principle stiffer, a smaller diameter less stiff (but not always in effect due to inconsistencies of density in wood). For practical purposes we assume a degree of consistency in a selected wood of a given diameter unless shewn otherwise.

It follows that in any given natural material there will be a range of spine in any given diameter.

If you know your draw weight at a given draw length and your safe shaft length there is a reasonable rule of thumb process for deciding on an initial selection of spine.

This is predicated upon the fact that a shaft of a given length will be of the same spine if it deflects the same amount under a standard weight suspended from the centre of that shaft.

How this can be usefully applied to the task of spine selection will probably make more sense when you understand these basic principles.

But the ultimate test is where the shafts go when you shoot them, and this will at some point require that you become able to distinguish between what the archer is making them do from what they, the shafts are trying to do.

Rod.

Thank you for your kind words. Never been called that before.... been called a lot of other things, not all of them repeatable.
I actually spend more time teaching than I do just about anything else, possibly to the detriment of my own shooting, I regret to say.
For the last few years I been running "instruction" at Sherwood Forest and elsewhere thinly disguised as a public access "have a go" and probably take more pleasure in seeing others do well than I used to in shooting well myself.
I have been out a handful of times this year and am pleased to report that I have not yet entirely lost my touch, despite a fairly casual attitude towards competition.
But making holes in 3D's and paper can never be a substitute for those halcyon days when Grandad introduced me to bowhunting small game, back when it was still legal here.

Thank you again.

Rod.

I have often supposed that the original intent of the traditional definition of paradox probably meant the same as my "shaft definition", since it is so obvious that we should point the shaft were we want it to go, but the old definition has been so often repeated in ignorance, and usually with illustrations (vide the illustration of archer's paradox in, for one example only , Hardy's "Longbow") showing what is so patently not true, unless we are meant to be shooting a weak (underspined) shaft that by some miracle straightens up and does not fishtail), that it has acquired over time the unfounded authority of repetition by the uninformed or the intellectually careless, when it in point of fact it is not supported by any sensible consideration of the facts, post Klopsteg, Nagler and Hickman.

***

I guess you will need to close your left eye at full draw if you don't want to deal with varying degrees of lateral offset at varying distances.
I note that on the public access shooting, folks who try shooting cross dominant can be as much as 20" to 30" off line at 12 to 15 paces.
If you make shafts to go where you point them and loose cleanly from a direct line of force through the shot, you should be able to shoot pretty straight at any reasonable distance and have to offset only for windage.

Rod.

And.....

Well if you draw it further it will shoot slightly "weaker" but the question is, in practical terms, how much difference will it make and how much does your inbuilt sense of body memory and intuitive shot orientation make an adjustment.
But snap shooting is perhaps another topic.

In brief, the formal answer is that more shaft flexing will make the shaft go slightly right (right handed archer).
Although at a longer draw the lateral "angle off" of the shaft is shallower relative to the bow, the "angle off" at brace is the same.
Tthe most immediately apparent change is in the slight increase in draw-weight and cast.

All else being equal the shaft will also go a little higher.

I cannot say which of these would be more obvious to you, but I suspect most folks would notice the increase in elevation before a small lateral deviation.

On the other hand drawing longer can take the anchor out so that you shoot a line converging with the dominant eye to target line, not
sharing the same lateral line.
If this were consistent and apparent then aiming off slightly would be in order, but the increment of offset will increase with the distance.

Whilst it is true that properly functioning intuition and concentration on the mark can accommodate many variables to a degree, it is also true that reducing un-necessary variables can make consistent accuracy more readily achievable.

If you understand why you do what you do, then you can make a more informed decision.

We may come to where we can just look and shoot, but will we be doing this with precision or more in hope?

Why would you draw longer? Your answer should inform the decision...

We can usefully talk about the trade offs implied here....

Rod.

Whilst it is no doubt true that there will be variation in stiffness, we have to think about how and why this occurs.

It seems probable to me that a greater range of variation will occur in rift orientation, if only because there is a wide variation in the run and spacing of of the grain.

A hypothetical case would be to propose several shafts of the same ring count and density, but with a wide variation of run off in the grain alom ng the length of the shafts with one perhaps being exactly parallel and with no run offs.

How do you suppose these shafts would vary in spine?
I would suggest that all else being equal, the more extreme the run off, the weaker the spine.
And that this issue would not arise in anything like the same degree with an edge grain (reed) orientation.

Having said this, I do not know of any successful competitive shooter, and I know many in more than one discipline, who matches his shafts to the rift.
When I last discussed this with my good friend F.Russell French, of whom you may be aware if you are an English archer who shoots competitively, he was of pretty much the same opinion.
That overall, arrows spined to the reed are laterally more consistent in flight.
Even in his best set, and he does make the most precise shafts you might hope to find, he will find a favourite shaft, which is harmonically more suited to the bow than most of the others, but this is a distinction beyond the ability of most archers to even recognise.

There are quite a few folks who espouse "unusual" theories regarding equipment, set up and the shooting thereof, but these are not generally supported by the results they obtain in open competition.
Not to say that it cannot be done, but it is sufficiently far from commonplace as to not support any contention regarding the efficacy of these notions.

We find often enough that the proven methods work best for the very reason that they have been proven in the shooting. Anyone wishng to gainsay this has only to go out and prove it by consistently showing that by use of their chosen method, better results are readily obtainable.

Having said this, it is your right to indulge in whatever preferences you might choose; is also something that you may or may not choose to defend in the only way that has any meaning, with the performance of your arrows in open competition.

FWIW

Rod.