Pulling a Chas is a term used to describe the act of a sim master beginning a sim with no particular idea or plan for how the sim will unfold. The approach typically begins with the sim master throwing out a vague introduction and seeing where the crew takes the story. As the sim develops, the sim master comes up with a broad end state for the sim based on the strands developed by the crew and works to nudge the crew towards it, while adjusting the details (and the end state) as events dictate.

The term originated in Trek Online to describe Chas Hammer's approach to sim mastering.