A.

1. With the ProtonVPN client open, click the Kill Switch button.  It will open a menu with three options:
2. Kill Switch Off
3. Kill Switch On
4. Permanent Kill Switch

We generally recommend enabling the Permanent Kill Switch.  If performance becomes an issue or problems arise as a result,

See: https://protonvpn.com/support/what-is-kill-switch/

Note: some screenshots I took, since it looks slightly different than the article.