Here's the script that goes into your one home page, with comments:

<!-- The script below must be in the head section -->
<!-- of your home page, as in this example -->
<script language="javascript">
   function fillMain() {
   //You must change page1.htm below to the name of the 
   //to appear in your "main" frame normally.
   var defaultPage = "page1.htm"
   //Location.search isolates any text at beyond a question mark in a URL.
   passedName = location.search
   lengthOfPassedName=passedName.length
   //If a file name was passed, put that page in the main frame...
   if (lengthOfPassedName > 0) {
      parent.main.location.href=passedName.substring(1,lengthOfPassedName)
   }else{
   //If no file name was passed, display the default page for the main frame.
      parent.main.location.href=defaultPage
   } 
}
</script>

Here's the script that goes at the top if each page, whith more explanatory comments thrown in:

<!-- The script below is required in head of each page to be framed -->
<script language="JavaScript">
   //If there are no frames here already...
   if (parent.frames.length==0) {
      //...Pull the file name (e.g. page2.htm, page3.htm) off the end of the URL.
      spot=location.href.lastIndexOf("/")
      thisDoc=location.href.substr(spot+1,location.href.length)
      //Set the URL of the main frame to the default page, or passed file name page.
      //To use in your own site you must replace index.htm below with name of your 
      //home page (the page on your site that contains the frameset and frame tags).
      location.href = "index.htm?"+thisDoc
   }
</script>

That, in a nutshell, is how it all works.

Alan