Backward chaining debugging follows the Prolog four port model. Backward chaining rules may be "spied" (this is a Prolog term which corresponds to tracing and single-stepping) which puts a break-point on them and means they can be single-stepped when they are invoked. When forward chaining debugging is on, the action part of forward chaining rules can be spied and single-stepped in the same way when they are fired. The Programming Environment, explains this in detail. The leashing of the ports can be adjusted, details are to be found in Debugging.
KnowledgeWorks and Prolog User Guide (Unix version) - 6 Dec 2011