Demos for new physics technologies usually look like Bodyform commercials, with gentle blue water sloshing around and lots of smooth rippling fabric. That's how Nvidia have shown their new unified particle-based physics tech PhysX Flex so far - lots of rubbery water balloons flopping about and leaking blue wet. Pssh, it won't look like that in the games we actually play.
As Nvidia explain, current games physics tech tends to run on a load of specialised modules, with separate techniques for rigid bodies, fluids, clothing, destruction and so on. Flex aims to unify all effects, materials and objects under one particle-based physics system, which will let them fully interact with each other. "For example," Nvidia say, "a water balloon made of rubber can be realistically destroyed by a bullet, the water within can burst out when the bullet strikes, and it can then cause the destroyed balloon to float on its surface." There's that blue water again.
With Killing Floor 2, it's more: a monster made of meat can be unrealistically exploded by a bullet, the blood and guts within can burst out when the bullet strikes, and the blood can then cause the destroyed guts to slip on its surface.
If you want a 'grey box, blue water' demo to help you understand quite what's happening there, here's an old demo video from 2014:
In short, guts. Killing Floor 2 is currently on Steam Early Access, where it shall be for a while yet.