Amentum

An amentum (Greek: αγκύλη, ankyle,^[1]) was a leather strap attached to a javelin used in ancient Greek athletics, hunting, and warfare, which helped to increase the range and the stability of the javelin in flight. Stability in flight was important because it allowed the javelin to land on its point, which was the only way the throw could be accurately recorded in competition or be useful against a live target. An amentum also increased the effective length of the throwing arm, as does a spear-thrower, so enhanced speed.

It is very similar to the Swiss arrow.