Excerpts from the book: 

Ten Gates; The Kong-An Teaching of Zen Master Seung Sahn

I thought that these writings were interesting and added some context to understanding the Kong-an tradition. 

From page x, A Note on the Photographs

Zen Master Seung Sahn meets a young monk of the Kwan Um School of Zen is the usual format for kong-an study:  student and teacher, face-to face.  As the interview unfolds, the student is asked to to demonstrate his or her understanding of questions asked by the Zen Master.  Sometimes an answer is in words, sometimes in action. Approval or disapproval swiftly follows, so the process is intensely lively and often humorous.  The student begins to see his or her attachments interfering with clear understanding.

Introduction Page xii

It was not until the Sung Dynasty (960-1279) that the exchanges were formally recorded and organized into collections, then commented on and used as a teaching tool by generations of Zen Masters.  by this time, because of the large numbers of students, Zen Masters were not able to have much direct contact with students in their everyday life situations.  The two most impoortant collects of kong-ans survive today and are still in use by Zen Masters as teaching tools.  The Blue Cliff Record (Chinese:Pi-Yen-Lu; Japanese:  Hekigan Roku was compiled in 1125 and was similar to the Models of the Elders gathered by the monk Hsueutou (988-1052)   A century later, the collection called Mu Mun Kwan appeared, collectred by the monk Hui-kai (1184-1260)

Introduction page xiv

In a country such as Korea with an established Zen tradition, students are not accustomed to seeking, nor do they receive, elaborate explanations about kong-an practice. Students typically encounter a Zen Master once, receive a kong-an and practice with it for years while having no contact with the teacher.  After much hard training, they again visit the Master to test their minds.  

Introduction page xv  

The word kong-an or "public case" refers to the Chinese custom of authenticating copies of public documents with a seal.

If you have copies of this paper elsewhere then you compare the seals to check whether it is a true copy or not.  So, if someone said, "I have attained elightment," then the Zen Masters use the kong-ans to check whether that was true or not.  They used it to find out whether the student had correct understanding.