A lot of good answers here, I also agree that the throw is a continuing process of progress, until you have reached your own most comfortable/accurate grip.
For some years now I have been thinking: How do we teach the children a "correct" way of throwing from the beginning? Myself playing Basketball when younger have my inspiration from the penalty shot. The grip is important. Concider this, the grip and "the slip" are opposites. The important thing to bear in mind is that when you "let loose", all fingers/handparts/whatever should be released at the very same time. And don't forget the "follow thru", arm/hand pointing at the object you just throwed at! To avoid the fact that a lasting part of your body effects the throw. Compare with putting in golf, you hold the putter with your fingers together, not letting an index finger stretch down the club because (yes, you can feel in more control of the club that way, but) it will affect your ball hit, and you are not supposed to hit the ball with a twist.
My friend who have played Kubb the longest time, like from 1989, holds the baton "from above" and the center of the grip is quite near the center of the pin, causing it to rotate quite heavy. As he have played that way for 25 years, he's quite accurate. Think "Shawn Marion threepointer"...
The palm under the baton. That have succesfully been practiced by Team Ekeby, leading to 12 World championship titles. You can see it in practice in their old YT-clips. I tried that for a couple of years, but to med it was too instable on 8 meters, I tended to throw a lot more batong just to the right/left of the kubb. I still use that tech within my short game sometimes, and I'll use it as a back-up when my now regular tech goes cold.
So what have I ended up with, for now? Regarding the importance of the grip/slip-problem related above, I'll grab the baton with my hand just at the same angle it naturally positions itself when letting my arm down, relaxed. I think of the baton as throwing a "balance point". At first, I balance the baton with my index/long-fingers to feel the exact balance point of the accurate baton (it varies, you know). If I want faster rotation, my grip is centered, if I want slow/no rotation I'll grip the baton towards the end. Though it's one tech, it's a lot of various throws. Think of all various hook-shot types you can come up with, sky-, baby- etcc hooks, or think of "Boris Diaw shots". I aim "the balance point" at the kubb, not the baton, then throws with all from no spin to a ton of it, depending of the circumstances.
Hope it makes some sense