Re: Heard this one on R4 yesterday...
frequency is extremely stable
as load varies (ppl using power) the the mech load on the generating sets changes, as more or less generating capacity is added/removed this also changes the loading, this causes stuff to spin up or down - F=ma and all that(we are talking very small amounts) - this is how demand is managed, control sees the frequency drop and brings more capacity online to meet it. the only other way is to.....
well use a medium to tell you _exactly_what the load will be in 2 minutes time
anything that changes the relationship between the load and the generating capacity will change frequency - that's what makes a 'black start' so hard to accomplish*
black start - bringing everything back from a catastrophic failure of the grid - the large fluctuations in load as circuits are switched back in and generators brought online tends to cause other generation to drop outside of its frequency range and therefore automatically disconnect itself from the grid, this makes the original problem worse, knock out more generation and so on and so on till we are all in the dark again.
This is why it takes to long to recover from a cascade failure, and i don't think anyone with any knowledge of the history of T&D could claim that this is an unknown phenomenon in the US :-D in fact when i think cascade failure the first thing i think of is the US! they have had a couple of doozies over the years
As far as this technique is concerned AFAIK it's been around for donkeys years (well.. ish) and is well understood, and pretty much impossible to fake.
(clue - your master blackhat recording tech when adding in his 'fake' hum will end up with 3 hums!, the original, his fake, and the one laid down when he did the fakery. immediately exposing the recording as doctored, and therefore unreliable.