1. A method of operation in an implantable cardiac stimulus device (ICSD) comprising a housing that contains a battery power source and operational circuitry that operates by analyzing cardiac signals from implanted electrode(s) in order to determine whether a treatable arrhythmia is detected, the method comprising: the ICSD making an initial determination that a treatable cardiac arrhythmia is occurring; the ICSD preparing to deliver therapy in response to the initial determination and, while preparing to deliver therapy, observing whether indications of benign cardiac rhythm are detected; when the ICSD is prepared to deliver therapy, applying Confirmation criteria to confirm the treatable cardiac arrhythmia is ongoing, the Confirmation criteria comprising a Therapy threshold and the Confirmation criteria is applied by: counting intervals between events detected from sensed cardiac signals that are shorter than a short interval threshold to generate a Short Interval Count (SIC); and comparing SIC to the Therapy threshold; and if the Confirmation criteria are met, delivering therapy from the ICSD; wherein the method comprises the ICSD changing the Confirmation criteria in response to identification of one or more of the indications of benign cardiac rhythm while the ICSD is preparing to deliver therapy by increasing the Therapy threshold to require a larger SIC value; further wherein the Confirmation criteria are actively updated and re-applied until either therapy is delivered or the ICSD determines that therapy is not needed; further wherein; the ICSD uses an event detection method in which events are detected from sensed cardiac signals, and intervals between the detected events are used to estimate cardiac rate for use in the initial determination that a treatable cardiac arrhythmia is occurring.