Why sliding expiration does not work for cookie less form authentication

i like to know in detail why sliding expiration does not work for cookie less form authentication ? i got a write up on this from this url http://blogs.msdn.com/b/carloc/archive/2006/11/11/don-t-slide-your-expiration-without-cookies.aspx but when i read the issue there just do not properly understand what they are trying to say.
so please some one explain it with example that why sliding expiration does not work for cookie less form authentication. thanks