Video by kyryllo
It's pretty clear that you need pretty good flexibility to perform the kick it is also important to know the propper technique in order to bring some force into it. It's not only rasing the leg and putting it on the target, if you are doing it that way you then can just leave it.
There are multipple ways to make an axe kick: stiff leg or chambered, inside, outside, front.
Stiff leg version(the leg stays straight while raising)is the more physicaly demanding, but produces more force than the chambered version, which is much faster though.
Untill the foot reaches it's highest point the hips should be square to the target the head should be upright and the back curved, the foot of the supporting leg should show forward.
In the stiff leg version the leg should be swung up high in a inside/outside curve or to the front. This should be done with the highest velocity and as high as possible. When the leg reaches it's highest point, the muscles and ligamets are tensed and stop the upward movement, this tension forces the muscles to contract which dramaticaly increases the downward velocity later.
In the chambered version the leg goes up chambered (bent) in an inside/outside curve or the front and is exetended above the target (hint: front kick above the target)
The key point in both versions is the hip movement. When the foot reaches it's highest point you push your hip on the kicking leg side towards the target, turn your supporting leg outside, and lean slighty backward with your upper body (physicaly you are not leaning back, your upper body stays in place but your hips are going forward).
This initiates the active! downward movement of the leg, the axis is not!!! the knee but the hip joint and the main muscles are the glutei, supporting muscles are the ischio-curales.
Be sure that the movement is downward and not to the side, if this is the case you are turning your hips and or/leaning back too far.
To learn the correct hip movement you should start with a slow kick hip high, and performing a kind of a pushing motion starting from the supporting leg through the hips, then the inside curved stiff leged version with a full curve (crescent), then place your hand where the highest ballistic point should be and perform the hipmovement as soon the foot touches the hand.
The outside curved kick is the most difficult to bring the hip movement in it, because of the alternating movement directions.