It uses WebPush notifications to set up a WebRTC data channel. So full E2EE with no website-run server in the middle (unless you need TURN). The actual domain is just a static site.
It is similar to a lot of the other WebRTC based solutions but it works better for me because the push notification means that I only have to navigate to the site on one device. Then on the other I can just click the notification. And other than initial setup there is no need to manage pairing codes or similar each time.
I mostly use it for between my phone and desktop. But has also come in useful for transfering files to and from my partner's Steam Deck and various other situations.
(Although last I checked it was having trouble on Chrome due to some weird interaction between the Chrome push server and the CORS proxy on Deno. It used to work and Firefox doesn't need a CORS proxy so that still works. I should probably get around to filing a bug against Chrome to enable CORS on their push server)