FCPX Transition Effect?

FCPX has a Draw Mask effect in the Masks category. Apply it to the clip connected to the storyline and click enough points* to follow the "shape" outlined in the video, and corner points to the edges (or beyond) the corners the animation is going to. Set the playhead at the beginning of the clip and click on the Control Points parameter keyframe. Advance a few frames and move the points following the outline that's animated to refit the shape. Continue (advance playhead, reshape mask) until you reach the end of the clip. Go back and move frame to frame and make any adjustments necessary.

*using linear segments are possible but will require many, many points. It's better to try and match the curves — Command-clicking on any point will drag out the control points for a smooth curve segment. Use a little feathering to help blend the edges with the background somewhat.

You got it! That's pretty much all there is to it: use a connected clip over the storyline and positioned to "animate" the mask where you want the "transition" to occur.