>>718143423
A kick moves your entire body mass behind it, anon. When you see a strong kick you will see the support foot turn, then the hips get twisted towards the target and all of that makes the leg whip into the target. If there was no target, it would go through and make the person spin or have to recover by turning back the hips and support foot.
What the gentleman in the webm is doing is light cardio, swinging just his leg. It's something beginners will do to gain balance, stamina and to build the neural pathways involved in kicking. Now, knowing all that, notice how despite that being the light cardio activity for people in their second month in the gym, that guy can barely do it for ten seconds without slowing down and losing balance.