"this" reference with buttonInstance.onPress?

I have a button on the main timeline, its instance name is “btButton”.

If I put:
“btButton.onPress function {this.attachMovie(…etc…);}”
nothing happens.

I need to refer to “this._parent”. Why?
I thought that since the button was on the main timeline, “this” should refer to the main timeline.

Again:
"btButton.onPress function {this._alpha=10;}
changes the alpha property of the button, while
“btButton.onPress function {this._parent._alpha=10;}” changes the alpha property of the _root.

“this._parent.attachMovie(…)” works.
_parent.attachMovie(…)” doesn’t work. Why?