Deleting clips
To delete one or more clips, first select them, then press Delete. Alternatively, click the trashcan icon 
on the timeline toolbar, or choose Delete from the selection’s context menu.
on the timeline toolbar, or choose Delete from the selection’s context menu.In smart mode, if the deletion produces a gap that spans all tracks, it is closed by shifting material to the right of the gap leftwards. This lets you avoid accidentally creating empty sections in your movie, while still ensuring that synchronization between tracks is maintained.
If the Alt key is held down when deleting, any gaps produced will be left unclosed.
In insert mode, gaps on the tracks from which clips are deleted will also be closed, but other tracks will be unaffected. No effort is made to preserve synchronization to the right of the deletion.
With regard to synchronization, the safest editing mode for deletion is overwrite, which will simply remove the clips and leave everything else unchanged.