Robin Debreuil mentioned about it in his nice new blog(Welcome Robin!) and further discussed in FlashCoders mailinglist.
Using __bytecode(“SomeByteCodeHere”); in your actionscript you can inject some bytecode into swf (It will be inserted at compile time).
It can be used to compactly represent some actionscript and hide the code behind in your FLA. But the swf produced is no different from that produced by normal actionscript unless you use some special optimization.
I decompiled the swf which had the script provided by Robin using both ActionScript Viewer and Kinetic Fusion and got the following script 🙂