If you have an extensive vocab, you should be able to think of the next word that rhymes with the previous one, and make sense.. you already have millions of words in your data base, its just the art of putting it together.. me personally, I will say a word, then whatever word rhymes with it, i say it, and make it make sense later..
i mean, freestylin should be easy to a rythmic person with a vocabulary, it's just incorporating punchlines and metas that makes it hard.. you need to already have a few sayings in your head already to really shine, or do like they do in the East Coast, change the definition of freestyle and spit a prewritten verse in one take, like Jay-z, they also do that in battles, any real hip hopper knows this.. so if you hear someone on MTV, or Freestyle Friday who spits an impeccable freestyle, with awsome punchlines and no mess ups, chances are is that its written.. unless you're like Eyedea, and talk about exactly whats in front of you at that moment, and rhyme with it..