These type of transitions are the easiest. Let me explain how
From frame 1 to 10 you have some motion tween - fading in.
Than from frame 10 to 20 fading out, or motion out, like in
your example motion out.
On that 10th frame there is an action that picks a variable
and go to frame in a
movie clip accordingly. All the button does is sets variable
the timeline, than timeline frame 10th action call the
content and while
the fading is done, the content has been changed.