A crisp double-haul will open more fly angling “doors,” allowing you to cast farther and more accurately, even in windy conditions. This comes in especially handy in saltwater and stillwater fishing, when you sometimes have to make long casts straight upwind.
Wisit C., host of the YouTube channel Angling Focus, has a novel but remarkably effective way of demonstrating proper double-haul technique, using nothing more than a strip of tape stuck against a blank wall. (He has a whole video on his tape method.) In this video, he covers the three most important aspects of the double-haul casting stroke: timing, length, and path. The slow-motion shots are great for highlighting each part.