[zachrip]

What does it actually mean to disconnect? Is there some physical connection being broken? How is that done?

[yuliyp]

Yes it's some physical connection being broken. It's not unusual to adjust the ways in which the electrical grid is connected. See https://www.youtube.com/watch?v=7Q-aVBv7PWM from Practical Engineering which talks about some of the nuances of building switches which can break connections between different parts of the grid safely.

[RvdV]

Yes, they are already keeping the link at 0MW transferred for a while and will now disconnect it. You can read more on Elering's website: https://elering.ee/en/synchronization-continental-europe