Trading paint means coming so close to another car that, well, in a parking lot you'd have to leave a note on the person's windshield. (When you're traveling at 180 mph, however, stopping to leave a note is inadvisable.)
Why would you deliberately bump into another car or brush against it? Basically, because winning a race demands aggressive driving. We'll go into the techniques of bump drafting and bump-and-run on the next couple of pages. Rubbing the car next to you means giving that driver some friction to deal with -- probably making him (or her) slow down, and perhaps letting you get into a more advantageous position. Imagine a foot race in which the sprinters not only run at top speeds but also throw elbows at each other.
It's possible to be too aggressive. Rising racing star Colin Braun was recently suspended for trading paint; he was already on probation for the same thing. He explains without apology: "I'm going to drive as hard as I can. Contact is always involved in racing, especially on the final lap" [source: Perez].
Is trading paint dangerous? Well, of course. Basically, you're causing minor car accidents on purpose, and at top speed on a crowded track, a minor car accident has the potential to become a major one in the blink of an eye.
The bottom line is, don't trade paint unless you're highly confident in your ability to control your own car. Even then, you have to gauge its appropriateness based on the track, the situation and the other drivers around you.
Trading paint and drafting are techniques that involve what happens around the car. Another kind of technique is all about what happens inside the car. Read on.