Nvidia FleX technology is a particle-based simulation technique that allows different simulated substances to interact seamlessly. It makes use of a unified particle representation for all types of objects and fluids.
Traditional visual effects use a combination of rigid bodies, fluids, cloth, ropes and smoke and rely on specialized solvers to work out how each substance interacts with the others. Nvidia said FleX's unified particle system allows all substances to interact with each other seamlessly. When Nividia displayed the technology in a demo, it showed liquid and smoke interacting with different objects that looked very natural.
Tripwire's use of FleX is a little bit messier. The company used the technology to help depict the blood and guts in a game. Killing Floor, as you can imagine from the title, is a rather violent and graphic game. The developers created a system they called M.E.A.T. (massive evisceration and trauma) that handles the dynamic gore, blood splatter and graphic violence, and FleX has been used for soft tissue and fluid interaction.
Nvidia hopes many more games will make use of this and other GameWorks technologies and has made the custom branches for Unreal Engine 4 available for free from GitHub to any developer who wants to make use of it.
Tripwire just happens to be the first to ship a game using FleX technology, but it certainly won't be the last. Just yesterday, Nvidia announced a partnership with Konami to help integrate GameWorks technologies into Metal Gear Solid V: The Phantom Pain. The new Metal Gear will rely heavily on stealth, and it's set in a large open world. Nvidia's engineers will be helping Konami's artists get the lighting and the special effects just right.