Waiting for the transmission.
Each five-digit group is read twice, as the real stations do. The transmission changes at the top of every hour and half-hour.
Where the groups come from
The groups are computed in your browser from the current half-hour and a few things about the visit: your browser and operating system, your screen size, your language and your time zone. The same browser in the same half-hour receives the same groups. A different visitor in the same half-hour receives a different message, because a broadcast is addressed to its recipient and the recipient here is derived from the visitor.
What happens to them
The page has no server-side state and no memory of you. Once it has loaded, the transmission is produced entirely on this device, and it works with the network switched off. If you want a record of what you received, copy it yourself. That is what reception is for.
Why the station cannot decode its own traffic
The groups are a hash of the visit, and the page says so. A hash is one way: the station can produce the groups from your browser, but nothing can turn them back into a message. Real numbers stations use one-time pads so that their traffic is indistinguishable from random digits. This station uses a hash so that its traffic is indistinguishable from a hash. The instruction to write the groups down is literal. Agents record; they do not need to understand.
About the real thing
Shortwave numbers stations have broadcast formatted numbers to intelligence officers since the First World War. Transmissions usually begin on the hour or half-hour, name the station and the recipient, count the groups, read them in fives, and sign off. The best-known recordings are The Conet Project, released by Irdial-Discs in 1997; the title comes from a mishearing of the Czech word konec, meaning end. This page keeps the same routine, including the sign-off.
Sources: the recordings at archive.org/details/ird059, and Wikipedia's article on numbers stations. The operator's test hook is ?slot=2026-08-03T14:00, which renders the broadcast for that half-hour.