Compound animations are a way to temporarily change an object's animation in reaction to events like spellcasting, fighting, etc.

The basic principle is:

an archetype defines a suffix, like "sword" or "spellcasting"
when an object does the related action, the code checks for an animation named "object's animation to which is appended the suffix"
if the animation exists, it replaces the object's animation for one iteration
if no suffix is defined, no animation is applied.

Animation suffixes are stored in objects in their obj::anim_suffix 's field.

Existing animation events:

when a player attacks with a weapon, the weapon is checked for an animation suffix
when an object casts a spell, the spell being cast is checked for an animation suffix
when an object uses a skill, the skill is checked for an animation suffix
when a player applies an item, the item is checked for an animation suffix.

From a code's point of view, the main function is apply_anim_suffix(), which does the work to apply a suitable animation.