It refers to a technique also known as "port knocking" which consists of leaving a port closed by default and opening it upon receiving a message by another channel (in this case, a UDP packet).
It was mostly in use when TLS hadn't made it's way into most common protocols
We still intend to publish the successor to Pithos at some point, it has seen a large overhaul in terms of how metadata is handled and relies on a in-house system for blob storage.
It was mostly in use when TLS hadn't made it's way into most common protocols