The Stretch transition expands the second video from a single vertical line to fill the frame.
The Stretch transition has 3 styles:
This style starts the second image from the center and stretches it across the frame.
The second video starts at the left edge of the first video. It then stretches to cover the first video.
In Push style, the second video stretches from the left edge, and the first video shrinks away at the same time, as if it was being squeezed out.
Swap video clips for this transition
By default this transitions starts from the first video clip and ends with the second video clip. Check this option to transition from second clip to the first clip.