The way to prevent a sling pouch from oscillating up and down, which causes the rolling of the ammo in the pouch back and forth, caused by cord seperation in the hand, as a result of the turning of the wrist during the rotor, is to totally eliminate the cord seperation in the hand, and eliminate the wide grip from the equation altogether. And instead, grip both cords together, with the release cord above the retention cord (side by side), so that their length during the rotor is not varied.
This seats the projectile very securely, which in turn allows a less cupped pouch, and a lesser chance of premature slippage of the projectile from the pouch. The prevention of the swaying of the ammo might also have a minimal effect on accuracy.
The generally accepted way to get consistent pouch angle control is to eliminate side ways oscillation of the sling cords and pouch (twisting.), and achieve taut cords that respond very directly to the wrist, and this is achievable by finding the right balanced projectile for your sling and through various technical means utilized in the sling's construction, or widening the grip and increasing the cord seperation in the hand.
Now my questions is, is the wide grip many of us use, including myself, merely a bandaid for a ''badly'' balanced and/or engineerd sling and/or projectile?
Many of us use a wide grip, the argument being it gives us a greater control over the pouch angle and a cleaner release. But what if we engineered our slings so that we wouldn't need a wide grip to get this pouch angle control and clean release we seek? What if we could obtain this control over the angle of the sling and a very clean release, even with a tight grip on both cords?
In my opinion this is attainable, and the most susceptible way to achieve both things seems to be through a rather wide corded, flat, sling. This type of sling, when balanced with the right projectile, has very little oscillations to the side, due to its twist resistance and it not being able to bend freely in all directions. This in turn allows a very tight grip, without sacrificing pouch angle control, thus eliminating up and down oscillations at the same time.
This might explain, why a wide grips are a rarity on the Balaeric islands, due to their flatter slings, and why wide grips are more popular with round and thinner slings.
Here is a quick video I shot, it's not about accuracy or power or form, but it's to show the tight grip I talked about and the flat sling in action. I wasn't convinced I could obtain this type of pouch angle control and clean release with a tight grip, until I used such a flat twist resistant sling. But I have yet to determine if I will stick to this tight grip, having used a wide grip for years.
https://youtu.be/zxEUiuom96k