[Reason077]

"Shutting off drains" is known as demand side response or DSR. It's often cheaper, faster, and more environmentally friendly to pay industrial customers to cut their demand by x MW than to fire up x MW of ancillary generation. And as far as grid frequency/balance is concerned, has exactly the same effect.

Often such customers will have flexible demand in the form of non-critical heating or cooling, pumps that only need to run some of the time, etc, which they are very happy to turn off temporarily in return for extra income.

This is indeed an automatic process, triggered in near-real time in response to signals from the grid.