Cover Traffic
The Nym mixnet generates cover traffic "loops" ... [l]oops generated by mix nodes ensure a minimum level of anonymity at all times, while end users can generate loops to obfuscate the timing and volume of their active communication through Nym and thus achieve unobservability.
In order to avoid timing attacks where an attacker tries to deanonymise which clients are communicating with each other, cover traffic is implemented. As soon as any client is connected to the mixnet, they will send Sphinx packets in 'loops' between nodes at a steady rate, inserting packets containing actual payload data (i.e. actual messages being sent between clients) when they are available.
In this way, Sphinx packets containing actual payload data are hidden in a steady stream of 'background noise' in the form of 'cover' Sphinx packets with empty payloads, but which are indistinguishable to an observer.
An in-depth explainer can be found in §4.6 of the Nym Whitepaper (opens in a new tab).