Circuit explanation needed

Hi guys,

Found this circuit on the web with an added description of how it works. However, being very new to electronics I can't understand exactly how the two back to back zener diodes come into the equation to make the bi-color diode flash. The bi-color diodes I own have three leads, A1 + A2 + common cathode but this circuit shows only two. Can anyone explain this and maybe confirm if this circuit will operate as described. Here's a link to a web page with the circuit and details.

http://www.williamhowarth.esmartweb.com/Circuit.html

Thanks in advanced

John

Thanks, this has been answered in the Analogue Design section,

John