To create this effect the interiors of the vehicles need to also be modeled in proper detail. For example, when the hood of the car is torn off, the engine compartment will be visible. You'll even see the engine's cooling fan rotating inside the compartment.
The detachable parts can also leave parts of themselves behind. For example, if the car's windshield is shattered, this will leave shards around the edges of the windshield. This level of detail creates a highly compelling feeling of driving in a car that's just short of a trip to the wrecking yard.
5. Damage textures
Even with the full car mesh deforming uniquely with each hit and car parts tearing off from the force of those impacts, the impression of damage is not complete unless the car also displays scratches, peeling paint and other damage to its paintwork. This effect is created by using damage textures, which are created uniquely for each vehicle to showcase damage that would be appropriate for this particular car.
When a particular area of the car is hit, the force of that impact determines not only how the car deforms and whether any car parts are torn off, but also what scratches, dents, grooves and paintwork damage is shown. The damage modeling system blends the car's texture between the pristine texture and the damage texture. The impacts on the car, the more of the damage texture is shown and thus more scratches and dents (and so on) are visible.
This blending effect provides a gradual increase in damage shown to the car and greatly complements the other major damage modeling systems.
6. Particle system
The particle system for each vehicle is responsible sending sparks, mud, exhaust gases and nitro flames in to the air as the player tears thru the tracks. As part of the car damage modeling system, the particle system creates the final touches for point of impact moments and the car's state thereafter.
When the car impacts with another vehicle or with a track side object, the particle system is responsible for sending car part particles flying through the air (e.g. bits of metal and glass shards) as well as fire streaks, mud, smoke and exhaust gas. This creates the effect of car shedding parts in the violent crashes.
After the point of impact, the particle system of the car is adapted based on the changed state of the car. For example, the car's engine block can catch fire and that fire can gradually become larger and larger thus increasing the size of the flames and the amount of smoke billowing out.
The detachable parts also affect how the particle system works. If for example, a muffler and the attached exhaust pipe are torn off, the exhaust gases and nitro flames naturally start to come directly from beneath the engine block.