Another great range tool feature (Apple)

The range tool trick referenced here?

You can also use it to speed/up slow down a clip. Just select the portion you want to go slower or faster - just the specific area you want to adjust inside a clip.

If you choose any value under 30%? I'd suggest changing the View Quality (from the menu category) to Optical Flow (an method to create synthetic frames for smoother slow motion.)

FCPX tips like this will be in sessions at NAB/Post Production World