On the last lap, you may turn and drive backward to defend. You are considered to be on your last lap when your teammates ahead of you can win the race by scoring and you therefore wouldn't be able to get another lap by continuing, assuming your teammates ahead of you all score.
The most common scenario in which this happens is that you have six players on your team, two in your hog, and you have just scored the 11th (or greater) point.
Let's do the math. You need 15 to win. Assume there are two of you in the hog and six players on your team. If your team score is 11 or higher after you score, then the four players on your team who are not in your hog would likely score before you, bringing your score to 15 for the win. You may use the last lap defense.
But what if you bring the score up to 12 and another hog on your team has already turned to defend? At that point there are only two of your teammates ahead of you to score, so you or your hog-mate must score to win the game. So you must proceed through your flags to try to score to help your team win.
What if you have three people in your hog and score the 11th point? You can't turn because there are only three people ahead of you in the rotation, meaning someone in your hog will need to score the 15th point.
Sound overly complicated? Then don't use it, or only use it in the 6-2-11 scenario (six players on team, two in your hog, and you just scored 11 or higher).
Once you have turned to defend, you must travel backward (opposite of the normal route) until you reach the vicinity of the previous base (usually red base), and then travel from base vicinity to base vicinity until the race is over. The vicinity is defined as within a few hog lengths of the base. If you miss a hog that you are trying to kill, you may not turn to go after them, nor may you slow or stop to give your gunner another chance to kill them.
On Gephyrophobia you may travel backwards (clockwise, as viewed from above) on the top level, without dropping to the lower level.
On Blood Gulch, Sidewinder and Ice Fields: you may travel base to base without passing through flags.
On Timberland, Danger Canyon and Infinity: you must pass through all flag areas as you travel backward.
On Death Island you must pass through all flag areas as you travel backward, except the small jump below the big jump.
You may not use the last lap defense on foot. If on foot, you must proceed through your flags in rotation until you can get in a hog.
~ No camping or sniping on Race servers
~ Respect the admins & all players
~ No profanity
~ No team nading/rocketing
Most of the rules are pretty easy to follow, but camping can have different meanings to different people. So here is the rule about camping:
Camping is defined as sitting and waiting in a hog, on foot, etc. for the intent of killing another player. If your life isnt in danger on a Race map you should be driving. If you have to stop to shoot to save your life this isnt considered camping.