Leashing allows the user to control execution while tracing for goals that are not spied. Spied goals cause execution to enter a debugging command loop whenever they are reached. Leashing provides the same functionality for unspied goals. A user may choose to enter a debugging command loop at any subset of ports by using (leash <events>) where events may be: call, redo, exit, fail. Leashing may be turned off using (unleash).
KnowledgeWorks and Prolog User Guide (Unix version) - 26 Feb 2015