Occlusion

Occlusion

Freecam vs a buried base

Freecam from inside the rock above a buried base. Vanilla sends all of it, Occlusion sends nothing the player can not see.

Freecam underground

Freecam deep underground looking down. Vanilla sends every cave, mineshaft and base; Occlusion sends none of it.

Entities behind walls

Mobs on the other side of the wall are not sent until the player could see them.