In almost every race there's a time when you shift gears from offense to defense. It usually happens when you realize that maybe you won't quite catch all those boats up ahead. And you start sneaking a few peeks over your shoulder.
It's typical to think about covering the boats behind when you're near the top of the fleet, toward the end of the race. This includes times when a) you're winning the race; b) you're not winning, but the boats behind are closer than the boats ahead; or c) you're protecting a position in the overall series.
There was a well-known football coach who once said that the best defense is a good offense. What he meant was that the best way to keep your opponent from scoring a lot of points was to score a lot of points yourself. I think the same is true in sailing. The surest way to stay ahead of your competition is to continue going fast toward the next mark. If you play the windshifts correctly and keep working on your boatspeed, there's no way that anyone will catch you from behind.
You can't, however, forget about your competition altogether. In fact, when your main goal is staying ahead of the boats behind you, their position will be the single most important influence on your tactics. In this situation, you just have to keep going fast and cover your competitors at the same time.
The most basic principle of covering upwind is to position yourself between your opponents and the windward mark. In other words, stay roughly on a line drawn between your nearest competitor(s) and the mark. This will minimize their chances to gain if the wind or current changes. In a windshift, the amount that one boat gains or loses to another is directly proportional to the distance that separates them. So when you're ahead, you want to minimize separation between you and the other boats.
Besides the general tactic of staying between your competition and the mark, there are a number of specific tactics that will help you maintain the lead. These include the "tight" cover, "loose" cover, and a combination of the two.
Tight cover
Using a tight cover on another boat is an aggressive tactic that allows you to stay between your competitor and the next mark, while at the same time giving them bad air! This not only helps you defend your lead, but may in fact allow you to increase it.
The most logical time for a tight cover is when the race has come down to you against one other boat. In other words, there are no other boats that you have a chance of catching or that have a chance of catching you. This is always the case in a match race, and oftens happens near the end of fleet races as well. Another time for a tight cover is when you're in the final race of a series and there is one other boat you have to beat.
Getting into a tight covering position is not always an easy thing. The reason is that in order to steal someone else's wind, you have to be lined up in the direction of their apparent wind, not their true wind. This means you have to be farther in front of them than you would normally think. A good way to figure out how to apply a tight cover is to look at the wind arrow on top of the other boat's mast; this will point right at the position where you should be.
If you're successful in putting a tight cover on another boat, chances are good they'll tack away to clear their air. If you can tack at the same time, you may be able to cover tightly on the new tack as well. Be wary, however, of tacking too much to maintain a tight cover. It's too easy to get off into your own little world and give up a lot of distance on the rest of the fleet.
Loose cover
When a tight cover isn't possible or desirable, you may want to apply a "loose" cover to defend against boats behind. To cover loosely, position yourself between the other boat and the next mark, but let her have clear air. You're not slowing this boat with your bad air, but you're staying in touch -- not allowing her to separate from you -- so she won't catch you on a windshift.
One obvious time to use a loose cover is when you aren't able to make a tight cover stick. Let's say you're crossing just ahead of your closest competitor on the final beat. If you tack right on her wind, she'll tack away, and you won't be covering her at all. It's better to place your tack so she still has clear air; then at least you'll have a loose cover.
Combining tight and loose covers
In the game of chess you have to plan your moves ahead of time and do everything possible to control your opponent. Sailboat racing is very similar, especially when it comes to upwind tactics. Here we can use a combination of tight and loose covers to influence other boats a lot more than most sailors realize. Think about how would you handle the following situations:
Situation 1: You're leading the race going up the last beat. You're on starboard tack, crossing ahead of your nearest competitor, who's on port. You have a feeling, after the first two windward legs, that there's better air on the left. Should you give this boat a tight cover, a loose cover, or continue on starboard tack?
You should probably give her a loose cover. Even though you think the left is favored, you're not sure enough to risk continuing on starboard away from your nearest competition. At the same time, you definitely don't want to apply a tight cover, since this would force your competitor to go the favored way. A loose cover keeps you on the favored side of the competition. If your opponent tacks to starboard, clamp on a tight cover to protect the left.
Situation 2: You're leading the race again, but this time you're on port tack, crossing ahead of the second-place boat, who's on starboard. The boat in third is also very close and is to leeward of you on port. If neither side of the course is favored, what should you do after crossing the starboard tacker -- apply a tight cover, a loose cover, or keep going?
Your main problem is that your two closest competitors are now headed in opposite directions. If this continues, it will be impossible to cover both. By putting a tight cover on the second-place boat, you can force her to tack so both boats are going the same direction. Then you should tack again soon to put a loose cover on both.
There are a few generalizations we can make about when to apply tight and loose covers. Put a tight cover on a boat that is a) headed toward the favored side of the course; or b) headed away from the rest of the fleet. Put a loose cover on a boat that is a) headed toward the wrong side of the course; b) going toward the other boats; or c) headed toward a layline. (Let them get to the layline as soon as possible.)