Cast your fly-line downstream (down current), so the artificial wet-fly is carried downstream and away from you by the current. The taut fly-line will arc across the current and swing back towards the bankside from which you are fly-fishing; retrieve the line with a series of short jerks to impart attractive, lifelike movement to the wet-fly.
When you have recovered all the line, cast again. Work your way steadily and methodically downstream (down current).
Downstream wet-fly fishing is sometimes the best way to reach otherwise unresponsive trout lurking at the water’s bottom, and is a useful technique when night fishing for trout, particularly sea trout – a wide area of water can be searched with a series of “blind” casts.