Overview
BeatLink refers to a family of systems in Resonite which are used to provide extra data from audio that users can use to add more interesting audio reactive effects to their items or avatars. This data is usually exposed through dynamic variables.
These systems usually use heuristics on the volume of the audio, to detect "beats" and provide information when those beats happen.
The accepted standards for how these systems communicate can be found on the Dynamic Variable Naming Standard page. Those standards should be used when making your own audio reactive effects, and are perfect for replicating similar effects from other games.