What a one-time link does
A one-time link carries something, in this case text you paste, to one reader. When they open it and reveal the contents, the stored copy is destroyed and the link becomes a dead end. It is the right tool for anything you would rather not leave sitting in an inbox: a password, an API key, a licence code, a private note.
The link is made in your browser. The text is encrypted there with AES-GCM-256, the key is appended after the # so it never reaches a server, and only the ciphertext is stored under a random slug.