When you’re either fishing downstream or have a lot of slack on the water—for instance when you’re making slack-line casts, such as a pile cast or a curve cast—setting the hook can be difficult. Because there’s a lot of slack on the water, if you simply raise your rod tip, all you’re doing is picking up the slack. Depending on how much slack you have between you and your fly, you might never even put any tension on the fly itself this way. No tension on the fly often means no hookup. That’s why you need the sweep set.
The sweep set uses the water tension on the line to apply pressure. You’re not trying to pick up the slack; instead you’re counting on the tension along the line to transfer energy down to the fly. Think about how the current pushing on your line makes your fly swing at the end of a drift. It’s the same principle at work in the sweep set. By pulling on the line, you’re kind of creating an artificial current.
To make a sweep set, sweep your rod horizontally toward the bank behind you, keeping the rod parallel to the water’s surface. At the same time, haul the line with your non-rod-hand. Always sweep to your downstream side. Otherwise, you’ll drag the line across your body, where it can get snagged by your vest/jacket or any tools you have hanging from it. Once the fish is hooked, you can raise your rod and start stripping like a madman to pick up the slack and establish a connection to the fish.
Because most of us have been taught to raise our rod tip to set the hook, employing the sweep set requires concentration and discipline. I usually make a mental note at the end of every cast to remind myself what I should do if I get a strike.