If you are with weapon on the back and walking slow and press Z the player made that quick movement
But only if you are walking slow, if you are moving "normal" like in the first try you do it correctly
I think that it is a bug
To test it on vanilla:
private _unit = player;
_unit action ["SwitchWeapon", _unit, _unit, 299];
With ace just weapon on back with "0" in vanilla:
private _unit = player;
_unit action ["SwitchWeapon", _unit, _unit, 299];
Then walk slow pressing "C" and while pressing W to move forward press "Z" and the character trys to suicide.
A video showing the two ways of walking with weapon on the back: https://clips.twitch.tv/HealthyProudPotKappa-AHUnhOMvog6x5E9y
a funny one: https://i.gyazo.com/9c38fb611eed716be1acde992c5dc08d.mp4 <3