This is a slightly more challenging variant of Dovetail Triangle that only opens when held in a certain orientation.

Watch the video for a demonstration:

http://www.youtube.com/watch?v=JnMRREjfPyc

Build Locking Dovetail Triangle1 and Locking Dovetail Triangle2. The pieces should fit together.

Choose to build 2 of either Locking disc or Locking triangle and then use them to lock the dovetail as shown in the video.

There are more pictures and an explanation of the locking mechanism on my blog at http://mysd300.blogspot.com/search/label/dovetail