In a lot of boats, pulling on the sheet is responsible for pulling the sail both down and in. When you pull the sail down, you are first of all affecting the leech tension. When you pull the sail in, you are primarily affecting the sheeting angle. As you progress, more boats will offer you the opportunity to do one without the other. Being able to adjust the one without affecting the other opens up a lot of sail trim possibilities, but even when the two are connected (Lasers, school boats), being aware of what you're after can help a lot.
C420, I420, and 29er sailors get to disconnect the two on the mainsheet by using a bridle, and windward sheeting the jib lets us adjust the jib's sheeting angle without adding leech tension. 29er jib cars get pinned in different positions to adjust sheeting angle. You can't adjust them in real time like a lot of other boats let you do, but it's a good start. Vang sheeting in a Laser, where the vang is tight enough to keep the boom down even when you ease the sheet out, is another example of separating the leech tension and sheeting angle functions.
Some SG sailors will remember a very light day this spring when I told some skippers to sheet Z420 mains directly from the boom. This allowed them to pull the sail in without pulling it down. The picture below shows what I was after there.
This is a TP52 and I'll use them a lot to illustrate points, both because they have the full array of sail controls and because there are tons of easy to find pictures that work for what I want to show. You can clearly see that their boom is above centerline here. As you go up the main, at some point the leech is just on centerline, and then towards the top it is below centerline. There are a bunch of reasons why you'd want to do this, prime among them would be wind shear* which we'll discuss later.
My red arrow in this picture points to the in/out jib track, which you can see just below the clew. The jib sheeting angle is every bit as important (often more so) than the main's sheeting angle. Notice also the main traveler position here - not even close to as high as in the pic above. I'd bet the mainsheet is so tight in this pic that the mainsheet winch is begging for mercy.
A tight leech is great in a lot of conditions - flat water and medium breeze prime among them. With a tight leech, you get to go fast and point high, but there is a tight groove*. Too tight a leech and the sail will stall*, cratering both your speed and pointing. This is why so many of you heard me say "when in doubt, ease it out" this spring.
A looser, twisted leech allows the sail to easily "spill" wind. This is great in light air to prevent stall, and also in heavy air to prevent excess heel. It also gives the boat a wide steering groove, which lets you go fast and point okay but lets you steer around and through chop without slowing down.
A great thought exercise while you are sailing is to think about which adjustment you'd prefer to make in different situations. Sailing along in flat water, fully hiked, and get a huge puff? Easing leech tension allows you to spill some of that excess power (there is also a thing in there about why Roy and I disagree so much about the truth of "ease/hike/trim," which is a topic for another day). Want to move bow forward relative to another boat? Keep leech tension on, ease sheeting angle a bit, and motor down over them. Big set of chop that you need to get through - twist the sails by having slightly looser leeches but tight sheeting angle is your fastest way through that.
Most of the boats you'll sail for the next couple of years don't give you the range of control options that would be ideal, but you need to start thinking about leech tension versus sheeting angle all the time. It's really important.
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