What else has changed for me is that I had originally learned to sling using a finger loop for an anchor. Research has turned up that the ancient Chamorros used a wrist anchor so I have changed my technique to include a wrist anchor. What hasn't changed is my throwing technique. Most cultures and slingers use a 'helicopter' style where the stone in the sling's pouch is whirled over head at a horizontal or tilted plane of rotation before one cord is released to hurl the stone. I sling this way. As for styles, I think that for the most part you need to find a way of throwing that works for you and keep with it. Consistency, as with with any shooting is key to accuracy.
So, the secret. Maybe it's not a secret to pin-point accuracy in as much as it is another style of throwing. By changing the anchor point on your hand, you can change the angle, rather the trajectory of your stone. Where this style works is using the same throwing technique and adjusting your anchor point to compensate for distance; close in or far out.
When throwing for distance you usually need a high arc. For far targets 50+ yards) I run the anchor cord through my index and middle finger like so;
For a flatter trajectory and closer targets, (30-60 yards) I run the anchor cord between my middle finger and my ring finger like so;
And for close in ground targets I run the anchor cord between the ring finger and the pinky like so.
My dad at 78 is still an avid slinger and he's been using this style of slinging, changing the anchor finger for distance since ever since. It's really not a secret in as much as I wanted to share it with you and preserve something passed on to me.