So first thing you'll need to do is a gradient mask using your favorite photo editing software. When creating a mask, all you need to remember is that white will allow the video to show through, whereas black will mask out the video. If you don't have any photo editing software, no worries, simply copy the example below to your computer to use.

Now, to recreate the first example as seen in my YouTube video, we'll first want to add another video track.
Select Settings > Track Manager... from the menu, click on Overlay Track #2 (which I'll refer to as track2 from now on), then OK.
Now, drag a video asset from your library onto Overlay Track #1 (which I'll call track1 from now on), and repeat for track2.
Resize your video clip on track1 as needed.
Now for video clip on track2, move it until it's just under the video clip on track1, and resize it so that it matches the size of the video clip on track1.
HINT: To make resizing and placement of track2 more accurate, click on the Enlarge icon on the bottom right of the preview screen.
Now in the options panel for track2, select Mask & Chroma Key.
Next select the checkbox for Apply Overlay Options, then for Type:, select Mask Frame.
Now click on the plus icon (Add mask item(s)), located to the right of the displayed masks and add the gradient mask you either created or saved to your computer. Then select it to use on your video.
Now let's also change the Transparency to 50.
Next, go to Filters and drag the Flip filter to you video asset on track2.
Choose Customize Filter, select the Vertical radio button, then OK.
And voila, you now have an interesting effect to add to your scrolling credits. In my sample video, I show another example of this technique in action. I'll leave that one for you to experiment.
Have fun and be creative!