Jauke,
I noticed this too, recently!
AncientCraftwork wrote on Jun 21
st, 2020 at 1:06pm:
Why is this sidearm throw of a biconical is rifled point upwards?
It didn't used to be a problem for me, but I changed my form a bit. A lot of releases are actually not rifled with the axis of rotation oriented in the direction of travel, the axis is tilted up. It has to do with the wrist orientation. Go through frame by frame in any of slinger's videos (pause and then use "<" or ">") and you can see that the way the wrist rolls through up until the throw makes the pouch need to yaw until it is oriented correctly. I seem to have changed the way I roll my wrist through ( I don't pronate the wrist as much as I used to), as well as the sling cords being less stiff, which doesn't allow the pouch enough time to yaw through to being straight on release, allowing for the correct orientation for rifling. Sorry if this is old news to everyone about the pouch rotation. It is something I always knew, but never thought about too much. I just always thought that I was getting the pouch to yaw the correct amount to achieve good rifling nearly every shot, I guess not.
I'm going to try a similar sling to what you did with the Y-sling, but backwards, 2 retention cords on the middle finger, to try and force the pouch to yaw sooner into the correct orientation.