The Circle was a custom of the Sardaukar which replaced the Sardau trial by combat with a trial by achievement. It was designed to vault the unpredictably appearing military genius over senior, but less capable, commanders. It also preserved independence and initiative.
Although battlefield obedience was demanded, following each action, the commanding officer faced a stringent critique from his fellow officers in the Circle of leaders.
Anyone — even the most junior — who could convince the Circle of leaders that he could have won the Objective faster, cheaper, or to better strategic advantage was chosen battle-leader for the next action.