http://code.google.com/apis/youtube/flash_api_reference.html
http://apiblog.youtube.com/2008/10/as3-youtube.html

The AS3 API allows users to control the YouTube video players by loading the player into a Flash application and making calls via ActionScript to the player's public API. Calls can be made to play, pause, seek to a certain time in a video, set the volume, mute the player, and other useful functions. The ActionScript API is automatically activated when the player is loaded into another SWF.