Load an external swf using this method?

A weird question:
I’ve always used URLRequest and Loader to load external swfs but today I tried this method:
there’s main.fla/swf/as
and asd.fla/swf/as
asd is of the asd class
and main is of the main class

in main.as, I used addChild(new asd())
and in asd.fla I drew a random square on the stage
but when I run main.fla, it does create a new asd instance (traced it) but the square doesn’t appear

is there some fundamental concept I didn’t understand here?