Double haul casts use the reserve power of the rod to cast farther and easier by increasing the line speed. Not only does it help arm fatigue due to casting all day, it also helps to provide the power in the stroke to combat headwinds, reach long distances, and handle large flies easier. The cast consists of four line-hand movements. The first movement is a standard single haul where you pull down sharply with the line in your hand on the start of the backcast stroke. The second movement is allowing your hand to drift up to the stripping guide as the line loop straightens out behind you. The third movement is to haul the line once more as you begin the forward cast. And the fourth movement is to shoot and release the line.
The haul on the forward stroke isn’t a long pull but a short, sharp one and is synchronized with the initial forward movement. You can provide extra line in your stroke by releasing the line through your hand on the backcast and forward cast. The extra line will load up the rod more for the release and shoot at the end of the cast.