The NREL inertia video explainer felt a little like it was begging the question - "inertia protects the grid because it has inertia and keeps spinning" - it doesn't quite feel like it explains where the extra energy comes from or goes, just that the mass keeps spinning. (I also haven't had a physics class in a long long time so some of this is not obvious to me, except that I understand from just common sense that if something's spinning you had to put a bunch of energy into getting it going in the first place and it's going to keep going if left on its own)

Anyway, I was hoping someone could fill in some details for me. Imagine a simplified grid: a dam that sends water through a penstock past a turbine/generator and into an electrical circuit, and a couple of resistance heaters on the other side of the circuit. The energy comes from water flowing through the dam - the dam operator opens up the sluice gate to let water flow through, the generator extracts the mechanical energy and turns it into electrical energy and it goes down the wire to the resistance heater where it gets turned into heat energy. Everything is balanced - the right amount of water is flowing through the dam to turn the turbine at the right speed to balance out all of the energy flowing through the wires and into the resistance heaters (and lost along the way, like losses in the transmission lines, etc). In this setup, there's some measure of pressure that turbine pushes back against the water flowing through the penstock of the dam, which is balanced out by how much pressure is coming from the water behind the dam and the pressure being put on the surface area of the penstock in the dam and the pressure being relieved by the water leaving the dam.

I get that thanks to inertia, if the sluice gate accidentally slams shut and all water stops flowing through the dam, the turbine is going to keep spinning for a bit and energy is going to keep going out onto the grid, though it will start to slow down due to friction at the turbine and energy being extracted from the system by the resistance heaters on the other end of the grid.

What I'm less clear about is how does inertia help when the water keeps flowing at the regular speed but when demand drops from the grid load. Let's say one of the resistance heaters turns off in a home somewhere - what happens to the energy from the water that was previously flowing into the grid via the turbine? Does the inertia in the spinning of the turbine somehow push back against the water flowing the dam, slowing the water down a bit/building pressure up in the penstock and behind the dam - with that pressure buildup being exactly equal to the energy that used to be going into the resistance heater? And that pressure either stays built up from the turbine until someone lowers the sluice gate a bit to cut back on the waterflow through the dam? Or does nothing involving inertia happen here - if the resistance heater gets turned off the overall load is reduced and the turbine spins a bit faster because there's less pushing back on it, and the water can move through the dam a bit faster, and the turbine just spins faster until someone notices it's going a bit too fast and the gate needs to be lowered so it drops back to rotating at 60hz?

Similarly, if someone turns on another resistance heater and now more energy is needed on the grid, but the sluice gate isn't opened up immediately, is inertia involved here somehow? If the turbine has to push harder on the grid side because of extra load, presumably the turbine slows down? Or does the turbine get pulled along by the new load somehow (more inertia?), and so more water can push past the turbine, giving it the extra energy it needs (and presumably dropping the water pressure in the penstock in the dam? And the pressure stays low until the sluice gate is opened up a bit more and more water can flow through the dam?)

I am using water pressure from a dam here, but I assume this would be equally true in a gas plant generating steam - if more energy is needed, the pressure in the steam drops until someone turns up the burner and creates more steam, etc, or if less steam is needed the pressure just builds up until someone notices and turns down the burner?

If anyone can explain how inertia and the grid translates into changes in the actual source of energy, I'd much appreciate it!