Use whatever setup is comfortable for your back layout. We are going to use a round off.
Flashkick
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Performing tricker: bam.defocus
The Flashkick is a backflip, where you kick over with one leg. It's quite easy once you have a comfortable
Back Layout.
01 Backflip
The Flashkick is a backflip variation, so you should be comfortable with it. It's possible to learn the flashkick first but the backtuck will teach you to jump up and to set, which is important.
02 Layout
Next up the Layout. As soon as you get this trick, the Flashkick is pretty much for free. Use a setup if you need to. My friend here uses a roundoff. Set up high with your body and spot the ground. Then drive both legs into the spot.
03 Flashkick
To turn your Layout into a Flashkick all you need to do is tuck one leg in and kick over with the other. You will set and spot exactly like you would for a Layout. Jump up, set, stick your chest out, look for the ground, push out your hip and kick your foot down into the spot while you tuck your other leg in.
The Set
Setting means to gain height before you initiate the main move. The Flash kick and the Back Layout set the same! That means you start out just like you would for a Layout and then you kick one leg to the ground when you spot the floor. Imagine a wall behind you that is about shoulder height. Then try to flip over that wall. Do not throw your body down like you would for a backhandspring. Also pay attention to the gif and where and when to spot. As you can see, you'll see the floor pretty much instantly as soon as you leave the ground.
For these you'll want to reach forward quite a bit, before your feet leave the ground. You'll get more power and blocking momentum this way.
Push off with your arms and swing both legs down.
Leave the floor with your body fully extended. Look up.
Imagine trying to jump over a wall behind you. Do not throw your body down to the ground like you would for a backhandspring. Start looking for the floor.
Spot the floor and push out your chest and hips. This will make your body slightly arched. Start your kick as soon as you see the floor.
Kick through! Tuck your other leg in to make way and drive the kicking foot down. Keep spotting the ground all the way through.
Keep going until you...
...land! Bend your knees to absorb the impact.
Tips
Drill Layouts first if you have trouble with height. You really need to set "over a wall" to have enough time for the kick come all the way around.
You should stick out your chest and push out your hip to make a slight arch at the peak of the flip. This way you'll be able to see the ground earlier and you can kick down with a whip-like motion.