How It Works in Caelus API

Caelus API registers a new entity attribute, caelus:fall_flying, on living entities. The attribute has a small base value that is not enough to glide on its own. When something that should enable flight is present - for example a worn elytra - a modifier is added that raises the attribute above the gliding threshold.

The bundled elytra modifier is caelus:elytra (value 1.0, added on top of the base). Each game tick, while the gliding condition holds, the entity's total fall_flying value is high enough that the engine permits fall flying; when the source is removed, the modifier drops away and the entity can no longer glide.

This is what lets other mods enable gliding from any source: instead of only checking the chest slot, they simply contribute to the caelus:fall_flying attribute. The result is normal, smooth elytra gliding driven by an attribute rather than a single hard-coded item slot.