＜JavaScript側＞
//objはMovieオブジェクト
//sendStrは渡したい文字列
obj.SetVariable('exFunc.call',sendStr);

＜Flash側＞
exFunc = System["externalFunction"] = new Object();
exFunc.onWatch = function(prop,oldVal,newVal){
  //ここで処理
}
exFunc.watch("call", exFunc.onWatch);