The SimInterface is meant to be a modular environment for constructing and maniupulating dynamical systems. In particular, the main task of the SimInterface is to handle bookkeeping tasks such as the value of internal signals, which variables get passed to which functions, and so on.
A good way to see what this is all about is to look at the DC Motor Tutorial. It goes through all of the basic features.
For the details of the algorithms, see the Documentation
This is a work in progress, so the documentation is still growing and changing.