Most trout in a pocket are not on the top. A nymph that reaches them, drifted once on a tight line, will outfish a dozen dry-fly casts at the same lie.
When
Anytime fish are not rising, and often when they are
Where
Pockets, runs and the seams beside faster water
Start with
A slim weighted nymph in 14 to 16, tungsten in fast water
See the take
A dry above it, or a tight line and a rod tip up
Watch a good angler work up a boulder gorge on the Eucumbene or the Kowmung and you will notice how little casting they do. A flick into the pocket at the head of a run, a second of slack while the nymph sinks, the rod tip lifted and led downstream at the pace of the current, a tick, a lift, a fish. Then a step, and the next pocket. Nothing about it looks like the fly fishing in the pictures, and it catches most of the trout in most of the pocket water in this country.
The reason is where the fish are. Most trout in a pocket are not on the top. They are tucked behind a rock, in the slack at the head of the run, or on the seam where fast water meets slow, eating the nymphs the current brings them. A fly that gets down to them on a drift that matches the water they are sitting in is the fly they see.
The rig is simple. A slim weighted nymph, tungsten if the water is fast, on a nine-foot leader. Then one of two ways to know when a fish has taken it: a buoyant dry above the nymph that dips on the take, or no indicator at all and a tight line you can feel it through. Fish the dry when you want a dry in play as well. Fish the tight line when the pockets are short and an indicator would drift out of the lane before the nymph has sunk.
The skill is in the drift: get the nymph down, keep slack out of the line and fish from the bank before stepping in. The fish you want is often one step upstream of where you stopped to tie on.
Fish it when the surface is quiet, the water is cold, or the fish are deeper than a dry will reach. The melt weeks on a snow-fed river, when the gauge is high and nothing is rising, are nymphing weeks.
Execution
Stand so your first cast is the best one. In pocket water the fish often sit at the head or along the soft edge, and a clumsy approach puts them down before the fly lands.
Drop the nymph at the top of the pocket, give it a second to sink, then lead it through on a drift that matches the current the fish is sitting in, not the current your fly line is in. Keep the rod tip up and the line off the water so the nymph fishes the seam, not the fast water beside it. One or two honest drifts through a pocket is enough. If nothing happens, take a step and fish the next one. Repeating the same swing through the same rock is how you teach the fish your leader.
On a longer run, start at the tail and work up so you are not standing in the water you still need to fish.
What goes wrong
The nymph is usually too high. If you never tick the bottom, add weight or shorten the dropper. If you hang up every cast, you are fishing the rock, not the seam beside it.
The second failure is slack. A belly in the fly line puts a second of delay between the take and the set, which is the whole take. Mend, or fish a tighter line.
The third is walking in. Pocket water is short. The fish you want is often one step upstream of where you stopped to tie on.