Step by step, basics: Turn around 180° bend the supporting leg a little stick your butt out watch over the shoulder the upper body rotates somewhat in the kick direction
hold your kicking leg loose but straight throw it in an arcuated motion stretch the supporting leg the hip leads the motion not the leg! bend down your upper body the angle between supporting leg and body should not be under 110°-120° the shoulder/scapula point to the target
shortly before the foot reaches the target rotate the upper body in the opposite of the kick direction while flexing the gluteus which give the leg the last bit of acceleration
when hitting the target the leg is straight after hitting the target you can bend the leg or just swing it out
Practice the kicking motion on the wall like it is shown in the video, while doing it make sure both of your hands are on the wall when you do the kick and release them only as soon as you "hit the target", other wise you could over turn the body which would mess up the whole technique. Don't swing the arms too much. If the basic technique works try to begin the kicking motion while turning around as it is shown around sec. 00:40 into the video, faster.