It's a javascript extension. i.e. you write javascript in a:

    <script type="text/sjs"> ... </script>

block, which is parsed and compiled into native javascript. It allows you to write asynchronous javascript using straight forward syntax. e.g. pause execution (hold), split code into parallel tasks, split code and automatically join when one branch finishes (function @ function - think search), and suspend/resume code using events.

It's very cool and now I'm going to start looking for an excuse to use it.