Well, in some way I think you are right to ask this question, and to make it easier for you, most of us did the same once upon the time.
To answer this question, or a statement, I think that all of us should have one fact on our minds and that fact is that slingshot sport and the accompanying market are still not so important and developed so as that governments and companies invest literally billions of dollar in research, like Space.
With that said, all we have by now is experience and some, I would say, limited experiments.
Let's take as an example width of the forks. Bill Hays took some effort and he showed that wider forks are more efficient than narrow forks. Why? Well, most probably because wider forks for the same length from the point between the forks and the center of the ammo is longer than with the narrow forks, This immediately means that the stretch of the same rubber length is longer, which immediately means that the stretch RATIO is longer and the in the same time ELONGATION is longer.
The first part - higher stretch of the same piece of active rubber, say 5.5 instead of 5 - means more power; but it also means more elongation. BOTH of these parameters affect the velocity of the ammo. Now, there are more questions to be asked here than that, but Bill made a solemn effort, he got some practical results and this is probably enough at this level; for more than that we probably need an institute - which we do not have.
There are other topics, such as what happens when you twist the pouch and, most probably, in this way you give some spin to the ammo - some answers do exist, most probably as the analogy with firearms or so, but truly, a twist applied to a slingshot shot is NOT thoroughly explored and I imagine that for that a fast frame camera is needed, some special marks on the ammo, some true ballistics experts and so on, which we do not have - but Discovery channel does..
So, at least at this moment we are doomed to experience, some limited experiments and analogies and personal preferences even if we had science behind us. Take this as an example: both Bill Hays and the science (if we had one) will tell you that shooting TTF and NO flip makes better results in the sense of precision, target shooting. But I grew up with bareback slingshots made out of old inner bicycle tires, which were so lazy that the flip was indispensable. Today, after 60 years of active shooting I find flipping a slingshot a cool thing to do to a such degree that if flips were forbidden I would then not shoot at all. Some people do it differently and I have no problems with that.
But your question is very important because it forces us to think about real inner workings of the slingshots, which takes time, effort and funds. Again, some people will agree to that, some people wont.
So, maybe in this forum you will not get much scientific answers as WHYS but if you ask and listen carefully you will certainly get valuable leads from people who maybe can not tell you why - because maybe they do not care - but still have a bit of valuable knowledge to share with you and all of us.
I hope this helps,