The Edit Page : Editing Your Project : Razor Blade

Razor Blade
The Razor Blade lets you cut or split any clip on the Timeline or Storyboard, with the exception of Transitions and Montages. This is useful if you want to remove part of the media without trimming, or if you want to add a cut so that you can replace one side, or insert another clip between the two sides.
To split a piece of media:
Scrub along the Timeline until the Playhead reaches the location within the Video or Audio Track (or both) where you want to make your cut. Tap the “Razor Blade” icon.
Choose one of the three options from the pop-up menu; Video (cuts only the Video Track), Audio (cuts only the Audio Tracks), or All Tracks (cuts the Video and all Audio Tracks).