It's a little tricky, considering at a certain point you're just being an ass by blocking on a straight. (You've just spun out and someone's closing on you by about 30 mph, seen that happen a lot.) But it can also be done as just simply a racing move, like tricking someone into going to the very edge of the track and getting in front of them at just the right point to force them to back off a little to keep position going into the next corner. Useful in situations where you know that someone's going challenge you no matter what, but using this you can at least keep them alongside you instead of letting them gain the advantage into the corner. Swerving and multiple blocks are a no-no though, and changing your line under braking is also usually considered bad.