A cardboard box (and put something in it so the parents won’t look at you funny)

I’ve spent some time reading the Wikipedia article looking for the relevant part, I guess I was 10 mins early (didn’t get the chance to see your comment before that). Here’s the (probably) corresponding video, the first video result when searching for the freestyle javelin technique, in case it helps anyone: https://youtu.be/52rvqtiBoow?si=RiLjhJG2ttv-0s1W

Next to upvoting, I’m just writing this to potentially get your comment ranked higher for this post.