One interesting thing about the "load changes the frequency" thing is that if you measure the frequency in enough places, you can actually see how large load changes "ripple" through the system.

For example, there was a big power outage in Florida in 2008 that caused a generator to suddenly go offline, and several orgs had a couple dozen power frequency meters running on the grid at the time, so they were able to make an animation of the east coast power grid "ringing" over the course of about 10 seconds as the load changed rapidly throughout the grid.

The animation for that is here: https://www.youtube.com/watch?v=bdBB4byrZ6U