1. When asking someone out and AIM shows that they are typing for more than 6 seconds normally implying a 'no'.
2. When you say 'hi/hey/whats up' via AIM and it takes them more than 6 seconds to reply, normally implying that they are leaving and are saying goodbye.
person 1: hey
(takes more than 6 seconds to respond)
person 1: (thinking to himself: aww, shit, he gotta go. DAMN YOU SIX SECOND RULE!!!)
person 2: hey, I gotta go. Sorry, see you later.