SpaceX uses pneumatic 'pushers' to separate the stage. After a few seconds the t...

SpaceX uses pneumatic 'pushers' to separate the stage. After a few seconds the two stages are far enough apart that igniting the second stage won't cause any real damage to the first stage.

Check out the video on this page for a good view of what that process looks like: http://www.extremetech.com/extreme/149741-spacex-falcon-9-la...