A little video about it  https://youtu.be/nkN9MkvZHk4?si=bcbpbdbnVe8os9o1

I tried a few things. So far I have controlled it with a movement of my hand. But it didn't work very well in the experiments in Switzerland. It was extremely hot. After Shooting for almost a day, I was happy to still be in the right direction. I was no longer able to control my hand perfectly. That's why I thought about it and came across this solution. It is easy to implement. Can be used reliably even under stress and exhaustion.

For me, there are three things that determine a rifle spin release.
1. Being adjusted to the harmonics of the sling
2. The sling is pliable enough to open properly
3. There is enough friction to impart sufficient spin to the projectile.

For example, I've recently adjusted to the harmonics of Egyptian slings, and now when I use a Balearic sling, the projectile flies nose down. Different slings have different harmonics.
A stiff sling will also hinder a stable rifle spin with light projectiles, since it cant follow the radius and open properly. This is easy to observe with a fully waxed sling before and after "annealing".
On the topic of lead, I find the bullet has to be slightly flattened in shape for sufficient spin to be imparted,(otherwise it can skid) but that's also because I use lower friction slings. (compared to say, a soft leather pouch)