A couple of weeks ago,
Signe Brewster at GigaOm has a concise explanation of how it works:
SpaceX paired a Leap Motion gesture reader with its Siemens NX computer aided design software and added 3D glasses, allowing a designer to shape the part with their hands in a 3D environment. They can't build a design from scratch, but they can take actions like modifying the shape of an object. Musk demonstrated in the video it is also a useful way to examine a design in three dimensions.
There's more on the process posted at the Leap Motion blog.
Here's the video.