Youtube flash detection

[shezam]

Youtube is doing some sort of flash detection using canPlayV9Swf() but I can't find for the life of me where that method is defined. Any one know what this method does. After doing its checks it seems to do the the fallback message if it can't detect Flash or you have javascript turned off;

<div id="watch-player-div" class="flash-player">

<div id="watch-noplayer-div">
<noscript>Hello, you either have JavaScript turned off or an old version of Adobe's Flash Player. <a href="http://www.adobe.com/go/getflashplayer/" onmousedown="urchinTracker('/Events/VideoWatch/GetFlash');">Get the latest Flash player</a>.</noscript>
<script type="text/javascript">
document.write('Hello, you either have JavaScript turned off or an old version of Adobe\'s Flash Player. <a href=\"http://www.adobe.com/go/getflashplayer/\" onmousedown=\"urchinTracker(\'/Events/VideoWatch/GetFlash\');\">Get the latest Flash player</a>.');
</script>
</div>
</div> <script type="text/javascript">
var fo = writeMoviePlayer("watch-player-div");
</script>

Help Appreciated. Thanks.