Animation events
Markers on an animation's timeline that fire callbacks at specific frames — a footstep sound, an enabled hitbox, a particle effect — so the timing lives in the clip, not in the code.
A sword swing should sound and hurt exactly when the blade is at full reach. Rather than guess that moment in code, the animator drops an event marker on the right frame; when playback passes it, the engine calls a function. Footsteps, projectile spawns and switching a hitbox on or off are all handled this way.
Unity's AnimationEvent and Unreal's AnimNotify are the well-known implementations. The payoff is that timing and content stay together in the clip — move the animator's keyframe and the sound follows, with no code change. The events are fired by whichever state machine is playing the clip.