sslh lets one accept both HTTPS and SSH connections on
the same port. It makes it possible to connect to an SSH
server on port 443 (e.g. from inside a corporate firewall,
which almost never block port 443) while still serving HTTPS
on that port.

The idea is to have sslh listen to the external 443 port,
accept the incoming connections, work out what type of
connection it is, and then fordward to the appropriate
server.

The protocol detection is made based on a small difference
between SSL and SSH: an SSL client connecting to a server
speaks first, whereas an SSH client expects the SSH server
to speak first (announcing itself with a banner). sslh
waits for some time for the incoming connection to send data.
If it does before the timeout occurs, it is supposed to be
an SSL connection. Otherwise, it is supposed to be an SSH
connection.