The Simulation System articulates dynamic elements of the scene via the Physics System. At its core there are 3 main components:
Drivers are mostly write-only. They take in values to know how to articulate the physics objects and constraints.
Stimuli are mostly read-only. They read values from given physics objects and constraints.
Brains are the controllers of the mechanisms. They use a combination of Drivers and Stimuli to control a given mechanism.
The Synthesis Brain exists for basic user control of the mechanisms, while the WPILib Brain facilitates controlling mechanisms over a websocket connection with a WPILib HALSim instance.