MPM Visco

The Visco MPM option is meant to simulate Viscoelastic material properties. Click the following link to learn more about: Viscoelasticity In short: Viscoelastic materials can show a behavior like a “fluid” and “rubber” at the same time. Depending on specific material properties either the fluid or the rubber component defines the final behavior to applied stress (stretching or compression).

The science behind Viscoelastic material properties is pretty accurate and is able to capture actual material behavior as you would normally see it in the real world. However, keep in mind even though thinkingParticles' FlowMPM Solver is based on physically accurate models, most of the time You would want to leave the real world physics behind.
Faster simulation times are achieved by having a low sub-frame step size with a larger gird size (less voxels). Such settings usually result in a more inaccurate result, but it might just be enough for a 3D animation project. In any case, dialing up the settings can give you a scientific accurate result in exchange for longer processing times.  

Stiffness - defines the overall “rigidness” or stiffness of the Viscoelastic material. As always, all MPM simulation are heavily dependent on Cell Size. A proper balance between Cell Size and Stiffness prevents explosive behavior or and simulation artifacts. Explosive behavior can always be counteracted by increasing the amount of simulation sub-step in the solver.

Volume Conserv. - one of the most important parameters to affect the look or behavior of the simulation. A theoretical value of 0.5 (not recommended) will keep the volume constant even when the material experiences stretching. Lower values will allow the material to freely stretch and distribute stress within the material's volume.

Viscosity - sets how viscous (or how syrupy) the material should be. Reminder: every value You set here is dependent on cell size, stiffness and volume conservation. Higher numbers will make the material behave more like thick dough instead of a honey like material.

Elastic - defines a stress threshold at which the material should become an elastic material and no longer act as a fluid.

Elastic Dynamic - defines how fast or strong the elastic transition behavior should be.