Power balance test mode implemented in some scan tools also monitors changes in angular rotation speed of the crankshaft in the same way as relative compression test does but does it when engine is running and makes calculations for power stroke of each cylinder. If certain cylinder has a problem then power output from that particular cylinder would be lower causing crankshaft to slow down (very slightly but detectable by scan tool or PCM software) when this cylinder is at power stroke. If power output is non existent altogether due to misfire it would set a misfire code. Although if power output is only reduced than misfire code won’t be set but power balance test would show reduced cylinder contribution helping to narrow down an area of possible concern.
Power Balance Test