If you look in the Maps folder of the flexproject that comes with the flex branch, there are different levels that display the different features of flex. Theres is rigidbody, cloth, fluid (surface) and softbodies at the moment. Smoke is said to come later. The balls you see is what flex actually is, a particle based physics system. The are other system that use particles for physics such as: https://www.vicogamestudio/projects/vico-dynamics
If you try to open one the flex static meshes you will see that the system “fills” the static meshes with balls to simulate physics.