Content is encrypted in your browser. The key lives only after the # in the link and never passes through the server. You can also add a passphrase or a key file.
The key goes after the # in the link and is never sent to the server. Anyone with the link can open it.
Checking the link...
Receiving uses up one of its allowed receipts; if it's a one-time link, it can never be opened again after you press this.