Everyone wants to be able to fly cast efficiently. When I say efficiently, I don't mean just merely casting with tight loops and superb accuracy. Part of being a good fly caster means that your fly spends more time in the water actually being fished rather than sailing back and forth through the air. In many instances, shooting/feeding out line while false casting is a big component of this because it allows you to extend your reach to quickly hit those more distant fish. However, if you're starting with just a couple feet of fly line outside of the guides and need to reach a target 60-feet out but it takes you fifteen false casts to get there......that's not optimal. Not only may the casting stroke itself need work, but how you shoot the line may as well. How to help remedy this?
When false casting, one way to reach out to distant targets much faster is by shooting line not just forwards, but also backwards. In addition to learning the double haul which helps build tremendous line speed, most folks will likely initially learn to feed out/shoot line in just one direction—on the forward stroke. While that's definitely a great skill to start out with, it's extremely beneficial to also be able to shoot line on the back stroke as well.
Feeding out line on the back stroke is no different than on the front stroke—simply let some line slip through your fingers as the loop travels behind you, then firmly hold the line once again just before initiating the forward stroke. Start off by letting just a tiny amount of line out on the back stroke to get the feel for it, then work your way up to what feels comfortable and manageable after some experience is gained. It's also a big help to turn your head and watch the loop behind you to both make sure the loop stays tight and your timing for the start of the forward stroke is good. You can even use some drift on the back stroke as you grab the line to help smooth out the transition into the front stroke and help achieve some greater distance, too.
With or without a double haul, from short casts to long bombs, being able to feed line out in both directions while false casting simply allows you to lengthen your cast more rapidly. This can be a huge benefit in some situations, especially when sight fishing where you may only have a few precious seconds to make your presentation. Even if not sight fishing, saving several seconds on each presentation really adds up throughout the day and means more time spent with the fly in the water where a fish can actually eat it. Practice the technique and perfect it!