Apple Final Cut Pro 7 - Manual (Page 773)

In the resulting effect, the media element wipes across the screen to reveal the next shot
from right to left.
Asset Wipe Transition Using Two Clips
In this technique, you drag an element clip that already contains an alpha channel into
the “Clip” clip well. This makes the track matte element in the Clip Alpha Matte clip well
unnecessary.
Note: This method requires a video codec that supports alpha channels.
To create an asset wipe transition using two clips
1 Drag the main element clip to the “Clip” clip well.
This clip contains the main media element as well as an embedded alpha channel.
773 Chapter 47 Adding Transitions