It's basically adapting the signal protocol to work on nostr using relays instead of centralized servers. KeyChat did something on their own but they told me that they based it on Signal as well.

The double ratchet means that it's both forward (back into the past) and post-compromise (forward into the future) secure, which means, even if your private keys are leaked, the attacker can't read your messages.

Give me a shout if you want to talk through it. There are still limitations to how good of a UI we can build (yet) but I'm confident it's going to be a huge leap forward for security of nostr DMs.