I'm not totally sure, but it seems like it allows you to peek at the ClientHello message at the start of a TLS stream without performing a TLS handshake or othe

I'm not totally sure, but it seems like it allows you to peek at the ClientHello message at the start of a TLS stream without performing a TLS handshake or otherwise interrupting the stream, so it can be forwarded with encryption to a backend server. For clients with Server Name Indication support (most of them,) the ClientHello message includes the unencrypted hostname of the server the client wants to connect to, so you could use it to route requests for different hosts to different internal servers in a virtual hosting/reverse proxy scenario (eg. requests for app1.com go to a different backend server than requests for app2.com) without Nginx having to decrypt the body of the requests at all.