Armalite has a Tech Note on it:
Also a mention at:
http://www.google.com/search?q=ar15%20%22rear%20sight%22%20play&sourceid=opera&num=0&ie=utf-8&oe=utf-8 (google HTML translation)
Armalight also has a nice note about many features of the standard AR15 design worry customers, but are in fact normal. It's a little condescending in tone.