Here are a couple of helpful web pages on the subject.
This page floats theories about the exact nature of the one-way valve, and that the cyst is not so much a structure of its own but simply a gap in adjacent tissues:
http://www.eatonhand.com/hw/hw013.htm
Here are some photos of mine. It's apparently termed "Volar"
http://www.eatonhand.com/img/img00036.htm
Here is one of the usual "the condition is ideomatic" sites:
http://www.mdbonedocs.com/tabid/20192/mid/37487/ContentPubID/743/ContentClassificationGroupID/-1/Default.aspx
