Virtual stiffness
A stiffly connected systems in machine phase can present to a site in the system
a virtual stiffness of some other part of the system that is stiffly connected
via some mechanism that gives some amount of mechanical advantage.
The virtual stiffness is the local stiffness combined with the by the leverage factor multiplied
connected non-local stiffness.
(wiki-TODO: Give concrete examples. Improve on the therrse dense unreviewed descritption.)