I made a video about controlling the curve you get during slinging. As we've discussed here quite a few times, you can control it with sling selection and wrist orientation these affect sling orientation upon release and ultimately control the spin direction of a sling projectile. I just wanted it to be made available to those who haven't read the previous discussions and show the effect irrefutably.
https://youtu.be/ESrvvBFcrWMI didn't realize while filming that I wasn't releasing the ball inline with the camera, which I wish I had done better, but I think you can still see the effects of spin on projectile curvature.
NOOC's videos with wet tennis balls show the different directions of spin quite nicely.
https://www.youtube.com/watch?v=4QEby...
https://youtu.be/ajzuWrYfrfkOne thing that I was going to add into the video, but didn't think the effect was quite strong enough to be seen clearly, was changing the retention cord position on your hand. This will also alter the neutral pouch orientation and can affect spin as well.