Short-lining is fishing with only 1 or 2 metres of fly line outside the rod tip.
Let us take an example of loch-style fishing with dry flies or static/slowly fished wets dibbled in the surface. The vast majority of anglers I have shared a boat with put it "out there". "Out there" being 10-15 metres away, where they can't easily see the flies, or how the fish eats them. They don't fish any more water, in fact they fish less. They fish less because they fish every 10 metres or so. And then they encounter the problem that the boat rarely drifts straight and drag occurs or they end up fishing the other side of the boat, where I'm fishing, and then they find themselves swimming in the lake.
Compare this to the short-line fisher, who casts his flies short, doesn't spook fish by lining them, doesn't have drag problems when the boat drifts to one side, and can easily cover fish by picking up and shooting line. It's actually very simple to fish, you cast 1-2 metres of fly line - let's say you have a three fly leader of around 20ft in length - your point fly could still be 20ft (leader) plus 6ft (fly line) plus 9ft (rod) plus 3ft (arm) ie 38ft away - which is actually too far! So you make a parachute cast and instead of lowering the rod tip as the loop unrolls, you keep it relatively high and most of our two metres of fly line becomes slack under the rod tip. The top dropper should be 10 or 12 feet in front of the boat and the point is another 10 or 12 feet further than that. As the boat drifts towards the flies you can raise the rod tip, any potential drag can be easily mended away, and when a fish rises elsewhere it is very quick and easy to cover the fish's path. You are not spooking the water, and you are allowing the fish to swim close to the boat, where you have a better chance of catching them.
Let's say that you are wading a trout flat in NZ or Tasmania. The short-line fisher fishes exactly the same, but maybe with placing those couple of metres of fly line on the water, and instead of casting to where the fish cannot be seen, and spooking them, fishes his or her way gradually towards these areas. Believe me, it's very much easier to make a shot, when you only have to pick up a short length of line, slipping line on the lift, shoot line in the backcast and deliver the fly, instead of either stripping the line in, and/or changing direction with a long length of line, or worse, making a collapsed loop presentation cast because you had too much line out to begin with (this is very common; I've even seen it attempted by novices (in competition) casting at accuracy rings, and you can be sure that if they do this at rings, then they certainly do it to fish!).
Let us shift our attention to running water... the short line fisherman closes the gap between him and the fish with stealth. He is fishing in drab colours, or maybe wearing Sexyloops camouflage, he or she keeps low and gets close to the fish. Longish leader, short line - here he makes parachute or collapsed casts to keep the fly closer still, usually within 20ft, no more than 30. This is not what your typical angler does - he attempts to cast further, and across ridiculous currents, that makes fishing drag-free next to impossible.
So here would be my piece of advice for you, if you are a habitual "line caster", for the next dozen times you go fishing, do not allow yourself to ever cast with more than 2 metres of fly line outside the rod tip. Believe me, most of my trout have been caught short-lining and I've fished an awful lot in my life. Try fishing the area between you and a maximum of 30 feet away with the fly.
There are times when you need "long-lining" techniques; if the fish are chasing muddlers, if you are fishing deep with sinking lines, if the fish are in a seam, wind lane or over a drop-off that needs a long cast. But I would say that the vast majority of anglers I've met, would be far better off learning to fish up-close - and I mean really close.
The ultimate trick, of course, is to place your flies with purpose; it's never all short or middle or long, but instead a systematic covering of likely water. It's just that usually it's better to wade or drift to a closer position, rather than to fish from a medium or long-range one. And then every cast - and I mean every single cast, should be a searching SHOT. You are fishing very ineffectively if you are only "fishing the water".
PS Today's POD is an old one!