Spinner is based on so called "virtual servers" bound to different
IP-numbers and/or ports. This works about the same as if you would run
many servers on the same computer.
If your computer has more than one ethernet interface, or if it has
support for virtual net interfaces, you can
have many different servers/configurations running on the default HTTP
port (80). Spinner is only server that supports this, without running
many server processes (thus increasing system load). It's actually one
of the few servers that can make use of many different interfaces on
the same computer.