I can't comment on the spine question, but here's Joel Turner clearly showing you can shoot with a thumb draw of either side of bow.
Albeit on the "conventional" side (right handed shooter, thumb shooting from the left side of the bow), he uses a fixed crawl so that the index finger avoids the shaft.
(He demonstrates both right handed and left handed bows in the video.)

 

I can't comment on the spine question, but here's Joel Turner clearly showing you can shoot with a thumb draw of either side of bow.
Albeit on the "conventional" side (right handed shooter, thumb shooting from the left side of the bow), he uses a fixed crawl so that the index finger avoids the shaft.
(He demonstrates both right handed and left handed bows in the video.)

So, thumb shooting with the arrow on the 'same' side, has the advantage of holding the arrow against the bow, whereas, shooting on the Med side, it can push the arrow away from the bow. I tried the Med without a 'button' and the results was an unplanned release. Dangerous to say the least. I will keep working on it.

 

(writing all of the below with reference to a right handed shooter, reverse for lefties)

The thumb draw is almost mechanically clean, so shooting on the left side of the bow is perfectly possible, you just have to account for the fact that conventional archer's paradox rules do not apply. The arrow will fly to its brace height direction rather than where it points at full draw. As in, the paradox does not occur at all. The arrow still bends, but it does not actually vibrate.

Slow motion:

https://www.youtube.com/watch?v=Lenfq9pcApY

So having a center shot bow (as Joel uses) really, really helps. The people in Mongolia who shoot on the left side of the bow have to aim to the right to compensate for their arrows not flying to the full draw position. Some actually draw lines on their knuckles to help them aim.

As for spine, that's a complicated question. Shooting thumb-on-left, spine doesn't matter as much because, as stated above, the arrows do not vibrate, so the arrows just have to be fairly stiff, but not to a precise, exact dynamic spine. If they are too weak, they can start to fly sideways, which is what happens with a fingers draw on the right side of the bow. Which makes sense, since the deflection of the Med draw is HUGE compared to the thumb draw. Using incredibly, unreasonably weak arrows for thumb-on-left produces a similar effect.

For conventional thumb release shooting (right side of the bow), center shot modern risers are very forgiving. Nothing difficult about how Joel does it.

But with wide-handled trad bows, spine can matter but not in the same ways as for fingers shooting. Getting the timing of the arrow's bending just right to clear the handle cleanly can be difficult (though by no means impossible) since the initial deflection off the thumb is so small. If one has trouble with this, the usual approach is to get arrows that are rather stiff, and slightly torque the handle counterclockwise to help steer them to where you want them to go. In other words, manual paradox, not automated paradox. A dynamic center shot system, as opposed to a static center shot. The spinning of the bow in the hand with kyudo is a very exaggerated example of this.

Further watching: https://www.youtube.com/watch?v=dTZkrmM5hx4

 

So, if I get the essence, the Med finger release tweaks the string sideways, causing the lateral 'snakey' path of an arrow, and altering spine reduces this? Where as the thumb, being such a clean release, does not tweak the string sideways, and the issue of spine is basically eliminated?

 

1) It's easier to use the pressure of the fingers to keep the arrow appropriately next to the bow when shooting with a thumb release, but you can shoot either side.
2) It's said that ancient archers, particularly in Asian countries, didn't care about spine at all. Only weight. Spine is a relatively new concept to kyujutsu archers, for instance.

 

A few months back I bought a Martin Hunter recurve, and began a long and sometimes confusing dance with spine/nock height, etc.

This could be compared to my first real bow, a recurve, and arrows recommended by the shop. I became quite adept with this bow. Going back to it lately and doing bare shaft testing, the arrows I was sold were too heavy a spine. But, I could still put arrows where I wanted them.

The rationale I received was that shooting the 'wrong' spine arrows increased the 'sensitivity' to small aberrations in form, causing some wild shots when the form was not faultless.

Thank you for the mention of kyujutsu. My nihongo is rusty, but that seems close to kyudo?

 

Here is a video I made a few years ago

 

Spine in general is weaker on a bow with no cutout, no matter the release type.

If spine is incorrect energy is wasted by the arrow trying to return to a straight path.

Wasted energy is then not transfered to the target.

 

Ryddragyn,

It had been a few months since I joined ArcheryTalk, and I had forgotten that I had to subscribe to get notices. I did this today. And I have received one other notification than yours. I had been sitting here thinking that the topic was of no interest to anyone. I am pleased to be wrong again.