Non-weapon equipment and body parts can't inherit time from parent animation
In vanilla equipment can inherit time from the parent animation of the actor wearing it.
Edit: this has since been fixed for weapons with #3778 (closed) but not for armour or skin body parts.
It's an obscure feature I think only some russian mods use. It works well for equipment but has limitations for weapons: walking and running time keys are inherited over attack keys, so animations keyed to attack times only work while standing still. I think it would be best if weapons and shields gave priority to upper-body animations instead, then animated weapons would work too. chitin_spear.zip This file adds a surprise to the default chitin spear.
player->additem "chitin spear" 1
Using a vanilla base anim file or one that keeps vanilla times, stand still while attacking to see its animation.