For more uniform semantics throughout KnowledgeWorks, the right hand side of KnowledgeWorks forward chaining rules are executed directly by the backward chainer, as is the default meta-interpreter for a context which has no meta-interpreter specially defined. When compiled with debugging turned off, in many cases the backward chainer can be optimized out leaving raw Lisp code.
KnowledgeWorks and Prolog User Guide (Windows version) - 26 Feb 2015