When players walk through a prop, or a car drives through it, the prop has no collision or the wrong one. Prop Collision Editor lets you build it by hand: boxes, spheres, capsules and cylinders placed over the model, or fitted to it.
Each part gets a game surface (metal, wood, concrete…) and what it collides with. You download the same .ydr or .yft with its new collision, and its .ytyp if you dropped it.