A big generating, transmission, distribution and consumption system operates in balance, and essentially if the load goes up the frequency goes down and visa versa.
If an event occurs that changes the supply or load, then the system frequency will start to rise or fall appropriately.
If a frequency excursion event occurs the only way to get it back on spec is by rebalancing the system, which may mean dropping generation, dropping loads, or (most typically) by adjusting the mechanical input to the generators. Either backing off the input, or cranking it up.
The bigger the system the more resistant it is to events as its got more rotating inertia.
The problem is that generating stations can only operate very close to their specified frequency, and most utility generators in big countries will drop off the grid at less than a 1Hz deviation. This is because if a power plant is run off-frequency then vibrations set in, which lead to plant destruction.
Power plants dropping off due to under-frequency events usually leadsto a cascading blackout.
Countries with lesser grids specify wider tolerances for example here in New Zealand the generation plant has to stay on-line down to 48Hz.
I would like to stress that the system, when looked at as a whole can absorb many small changes in localizied load as they would not show up as even a blip on the radar screen, in the big scheme of things.
If units start to drop off, because of local load swings or failures, it can have a cascading affect on the whole system, whereby other units can now start to go through parameter swings and cannot stabalize. This is the study of transient stability.
It used to be that the US system was very steady and reliable. But to try and reduce operating costs, utilities have basically used up the spinning reserve, making less units on-line and available to pick up slack. On top of that, many of the units are at the end of their useful life, and need either major overhauls or replacement, making the possibility of failure more real. I suspect that there is a rather high likely hood of the number of uncontrolled blackouts increasing in the US.
What the others are pretty much saying, and I agree with, is that the answer to your question is: it isn't. In an emergency, generating stations go offline to protect themselves which really just makes the problems on the grid worse.
To simplify, the US grid can be summarized to be in one of two categories at any given time:
1. The frequency is 60 Hz plus or minus something on the order of 0.1 Hz, and all is well.
2. The frequency is more than 0.1 Hz in either direction from 60 Hz, and we're screwed.