If the grid is very slightly over 50hz it means demand exceeds supply and vice versa.

By automatically switching off the pump on thousands of fridges for a few seconds, networks would require less generation capacity on standby - saving $$$ and CO2.

Of course, like you say, this would leave potential for this to be used in reverse to destabilise an entire grid.

Not to be too pedantic, but it's actually the opposite. Frequency drops as demand goes up, which requires more supply to compensate. This is because the generators physically spin more slowly as they are put under more load.

Also occurs to me that it's 60hz in North America, but point roughly remains!