The Scissors Tool breaks routes, tracks, or shapes into two parts. The primary use for this tool is to clean up track logs from older GPS receivers, which often have a long, straight line connecting two recordings that you would like to treat as separate tracks.
Clicking on the offending line breaks the track into two tracks. Contrast this behavior with the Split command, which splits (or slices), rather than breaks, a track into two parts. Using the Split command results in two tracks, with no visible gap between them.
Using the Scissors tool or the Break command results in two tracks, with a visible gap between them.