You can begin on a laptop and continue later on a tablet with the same practice state. Device sync covers profiles, practice data, your own pieces and custom sounds.
Klavissimo encrypts the state on your device before it reaches the server. Only an encrypted container is stored there; the key stays on your paired devices. If two devices changed independently, a conflict dialog asks which state should win.
Plus sets it up; another device joins for free
Setting up device sync is a Klavissimo Plus feature in the browser version. Another device of your own can then join through a QR code or link that is valid for 15 minutes, without needing a second Plus key.
A file backup still matters. If every key is lost, Klavissimo cannot reopen the encrypted state either.