Olde-timey "DISSOLVE" thing goes here. Your brain fades back to a kinder, gentler time when normal terrestrial radio...existed. Before XM. Before Sirius. Before wireless, hands-free cell phone calling in your car. Before iPhone podcasts. It was the time of radio, and the power ratio had to do with, more or less, the "power" of that radio station's brand inside of a given market.
Think: Los Angeles. There are 27 radio stations in the broader DMA of the city and county, and let's say that, in the total region, over the course of a year, there is $500 million spent on radio ads (yes, this is a bygone era; today the numbers are way lower). So the CBS/Infinity Group has 20% of the ratings, or total Gross Ratings Points, as audited either by Nielson or electronically. So then you'd expect 20% of the total market's revenues to innure to CBS/Infinity, right? That'd be $100 million in revenues to that station group. Well, if that were the case, then the group would have a power ratio of 1.0. But, in fact, CBS had awesome sales people, cool tricks up its sleeves to broadcast from auto dealer parking lots, and Howard Stern, who goosed dollars always. So, in fact, in a given year, CBS had a power ratio of 1.3, which meant that it over-indexed ad sales in that Los Angeles DMA to gain $130 million of ad revenues, taking away share from other, weaker stations.
That's what a power ratio is: an index of how well you're monetizing your product. Common among media companies in the era, before the internet came along and, well, basically stole the copyrights of television and film companies and music libraries, and then gave them away for free to the masses. But that's a vastly different story.
