[tk75x]

"booster in the form of a leather thong, called an ankyle, that would set the javelin spinning, getting it to fly further."

Sounds like they just used it to wrap around the javelin to give it a spin like you would a spinning top with a string.

[pseudo0]

Neat, that does make more sense actually. Pretty impressive that the spin from a piece of leather would add 58% to the flight distance though.

https://en.wikipedia.org/wiki/Amentum