This intriguing mechanism features a constantly rotating input (orange gears) driving an output gear (small white gear) which rotates first in one direction and then in the reverse direction. This is shown (not very well) in this video: http://youtu.be/_0XB8Rn9RqA
Shown much more clearly in this animation: https://youtu.be/jA5n5GlCb5U which was created in Blender 2.76 using the Physics engine. If you look closely at the animation you will see that the trailing tooth of the central gear, and that of the ring gear have been reduced in height. Unless this is down the physics animation won't work BUT the animation is using a planet gear of 31mm diameter. I've uploaded a planet with an O/D of 29mm which is a bit sloppy in its engagement with the other gears but probably won't require modification of the trailing teeth. The design was inspired by this video: https://www.youtube.com/watch?v=3aobPgGzB-U&list=PLB0IDKx1j2rxVpbfnvui1v5lDLPlaB2gx&spfreload=1

Designed in MOI. Printed in Diamond Age PLA on an Ultimaker 1

The stl files should all have the 'correct' orientation for printing - but check!
I used Brim for the tall upright pieces.
I used Support for the couterbores in the base, but not essential.
I found it necessary to clean all the gear teeth where they had been in contact with the build plate.
After cleaning, I assembled the handle with a soft hammer.
NB The assembly requires 11-off 3mm Button Head Cap Screws with 12mm thread length, though you can probably get by with 10mm long screws.