In the ProtoFlux FrooxEngineContext, a Field Drive node is a listener node. Whenever the node is notified of a change from its input context, it will evaluate its input and write the result to the field. This makes ProtoFlux drives lazy by default--usually only evaluating their input when needed. However, as with all listener nodes, if the input node graph has a node marked with the ContinuouslyChanging property, it will cause the drive to evaluate its input every engine update.