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.