Step 1: The Push
The time and patience to learn.
Hold the pen like pictured in the image so that the index middle and thumb is in a triangle while you hold the pen between your index and thumb.
Take your index and release it from holding while you push with your middle finger remembering to keep your thumb straight.
Step 2: The Catch
As it comes around from the intial spin back to where you started just take your index finger and try and catch the pen in the same place you started from.
If you require more help or wish to further learning more tricks ( yes there are more =P) go to upsb.info and register on the forums.
Video of the trick