proper Axe kicking
Imagine that, brought down with the heel, into the ribs or solar plexus of a downed opponent. Only the rear tip of the heel connects, making a very localised impact, much more likely to cause real tissue damage. A properly executed axe kick involves hip rotation to pull the leg in, so its not just aobut leg power, the hip torque snaps the leg down generating insane amounts of force.
I can understand you having never seen an axe kick KO, because very few people can do them well, Cro Cop has done one or two good ones, Andy Hug was very good at them. Glaube Feitosa is very quick with them, but he doesnt turn his hip and finish the kick looking side on at his opponent, he just continues to stand square on with them, so his arent very powerful. I must admit, Ive never seen an axe kick KO either, but then they arent often used to target the head, they are more often used to hit the trapezoids that are exposed next to the neck. Getting those hit is incredibly painful and makes fighting on very difficult, as they become stiff and even minimal head movement because agonising, I've pulled my trapezoids before and I spent a whole day lying down, unable to move my head, I can only imagine how bad it must hurt getting hit by a full power axe kick in one of them!
I always thought the best place to use them was when the opponent was on all fours on the ground, where many competitors in pride used a soccer kick to the head an axe kick to the opponents ribs on the far side of their body could have been abosolutely devastating!