The “double haul” is when you use your non-casting hand to pull (and release) the fly line on the forward and backward casting strokes. Doing so adds flex to the rod, which builds line speed. Faster line speed creates more distance, as well as ability to bust through the wind.
Here, casting guru Brian O’Keefe shows that the best way to learn to haul is by making some casts with the rod parallel to the ground. Know your start and stop positions, and as the rod swings forward to back (and vice versa) make a single haul by gently tugging and releasing the line.
You’ll build a feel for the haul, going “down-up” with the tug, and you’ll be able to move the casting plane to a normal, upright position. Eventually, you’ll shoot line as part of the double haul… “down-up-shoot… down-up shoot.” When this becomes second nature, you’ll double haul instinctively, cast straighter, and cover more distance.