This is a gainer flash kick using a j-step setup:
So a gainer is a back flip taking off one leg with momentum, and a gainer flash kick is just the same except you kick and land on that kicking leg. The j-step setup is just an angled entry that maximizes the return you get from the momentum you put into the trick.
I recommend having a back flip. I feel that a flash kick is really helpful too, because on a flash kick you don't tuck, and getting used to that is helpful for the gainer flash. If you don't have a decent tricking background, and cannot backflip, you can still get this trick. I've known a few people who've gotten it before they could standing tuck. They just looked over their shoulder a bit to cheat the looking-backwards-thing that's scary. If you don't have the prerequisites for this trick you can try it anyway!
A natural progression would look like this: Within a month of landing your first back flips you should be able to do flash kicks. When you get your flash kick, you should be feeling capable of attempting gainer flashes within a month. Just start trying backflips with a pixie hop setup. This is also called a reverse gainer setup. See the first clips in the above video. Then, when you get the feel of that, start trying to kick and land on one leg.
When you start to get this trick, you might be confused about what is actually making the movement work. I encourage you to try this trick on your other side too, to help you understand the play of forces working to make the move effective.
The best way to improve this trick is to do it quicker. Less hesitation at takeoff, shorter foot-ground contact time, quicker kick, shorter amount of time between each piece of the trick. Again, I know this sounds like a lame tip, but it's true. You can spend a lot of time messing with angle of entry, alignment, direction of force etc; but you will probably have the best results simply trying to speed the whole thing up. And speed just takes practice.
As you begin to speed up your gainer flash, begin working on entering it with more momentum and and learning to control this additional momentum. This is the last thing you should do (in my opinion) to improve your gainer flash, more momentum is the finishing touch. Watch out for wet grass too! And glass. Watch out for wet glass!
The gainer flash is one of the tricks I've kept since I've gotten it. After long breaks from tricking, it's always one of the very first tricks I return to. I feel this is one of the most simple, valuable, satisfying and beautiful tricks there is. It's my #1 trick to do for non-tricksters, because it seems to impress them a lot while distinguishing what I do from gymnastics and martial arts; It's also the easiest trick to do for that purpose. This trick is also useful for combos, has a lot of variations, and serves as a prerequisite for a lot of tricks. It's safe, and a good warm up trick too once you build it up. If the 540 Kick is the king of kicking tricks, the Gainer Flash is the king of flipping tricks. You want this trick.