The amentum isn't the super-weapon I've read about on a lot of forums. I've experimented very heavily with them, and at best I get a modest increase in accuracy and power. I don't get any increase in absolute range, what I do get is a modest increase in effective range, as the point stays on target longer and the spear/javelin seems to be impacting with more force. One thing I will say is that the amentum isn't something you can pick up and instantly be better at throwing javelins, it takes almost as much time and practice as throwing the javelin by itself.
In my mind, though the principal is the same, the swiss arrow is a different beast altogether. It's a fun beast to work with though, and I have made ones which work similarly to the one in the video.
So men who are free
Love the old yew tree
And the land where the yew tree grows.