I find it difficult to believe that anyone can flow off the top of their head and execute it perfectly. And I mean 100% off the top of their head. Even personals are thought about in a battle. If you were the 2nd to spit in a battle, wouldn't you be sitting there thinking of topics to diss your opponent? No one just sits their staring and then gets up and magically has 20 lines of personal disses towards an opponent. I'm not saying rappers don't flow off the top of their heads, it's just that every thinks one step ahead (when your rapping one line, you thinkin of what you'll say next) and pre-thinks.