Gaps are spaces in a layer that do not contain media. Create gaps to see through one layer to another or to apply soft effects to the layers below the gap. If a gap exists in a layer, the next layer that contains video media is included in the edit as long as it is also below the focus layer.
By default, all gaps are transparent. However, you can make a gap opaque if you do not want to see through to the next layer. You can only make gaps that do not contain soft effects opaque.
Also see Creating Gap Effects.
