In my opinion free style rapping isn't just something that a specific person can do. It does take time to make up a nice rap that makes sense and sounds good. What you say doesn't really have to do with a specific thing it just has to rhyme. The only people who probably can free style rap are people who listen to rap music just about every day. I think anyone can make up a small rhyme within a couple of seconds, but not every body can just sit back for a second and then come up with one verse or two.
Sometimes the only way to make up a rhyme is to cuss or put names in your rap from the people around you. If you get stuck at one point, you can always take a line out of a song and change it around in your own way
Sometimes when I'm free style rapping I talk about school, today's society, other rappers and myself. Once you get these things down you are ready to free style anywhere and anytime.