What that practically means is power that would have normally come through those substation would be delivered by some other source, and thus go through a different set of substations.
What most people don't understand is that it's important that the demand + the transmission losses = the production, not just as a whole, but also for each link. To help with that most places don't produce 100%, and a lot of higher power links are redundant. But that can only cover so much. If it's not a high demand day you can probably source power from other providers, but sometimes that's not enough.
EIL5 Answer. Power (mostly) follows the path of least resistance. Like water does.
To divert power certain stations or links in the grid can offer resistance via either terminating, or other means.
Another thing is, It’s a common understanding that power facilities can produce and modulate power. But they can also spin in reverse, thus using power.
What most people don't understand is that it's important that the demand + the transmission losses = the production, not just as a whole, but also for each link. To help with that most places don't produce 100%, and a lot of higher power links are redundant. But that can only cover so much. If it's not a high demand day you can probably source power from other providers, but sometimes that's not enough.