So its a round kick, with a stiff leg that comes down onto the target from above.
Usually down onto the head, or leg.
1. Start kicking when you're six and don't quit. You need really good hip flexibility to get the leg up and over like that. 
Are you aiming at the leg or torso?
One requires you to bend your leg at the knee, the one supporting your body, the other requires your hips to swing more than 180 degrees if your kicking with your back leg.
Both kicks should have your supporting foot rotate as far as 180 degrees upon completion to generate whipping power from your hips. 
if your aiming for the leg, what i have learned is to fake a knee, then come down with the kick. Tried it in practice and it hurt them like a bitch, although it doesn't look like it does.
Best i could think of what you're trying to aSK
man meets fate - if your aiming for the leg, what i have learned is to fake a knee, then come down with the kick. Tried it in practice and it hurt them like a bitch, although it doesn't look like it does.
Best i could think of what you're trying to aSK
thats great advice do you mean an upwards knee strike comming straight fowards? Comming up at a 0 degree angle.
Or a diagonal knee strikes comming up at a 45 degree angle?
Could you fake the teep and do this downward kick?
alexkid21 -man meets fate - if your aiming for the leg, what i have learned is to fake a knee, then come down with the kick. Tried it in practice and it hurt them like a bitch, although it doesn't look like it does.
Best i could think of what you're trying to aSK
thats great advice do you mean an upwards knee strike comming straight fowards? Comming up at a 0 degree angle.
Or a diagonal knee strikes comming up at a 45 degree angle?
Could you fake the teep and do this downward kick?
hard to explain it. Here is the video of what i'm talking about
starts @ 3:58
i think that's similar to what you're looking for possibly.
also, i never tried faking teep and coming with the kick. Seems too risky, But faking the teep and coming in with a knee works pretty well.
Helps me to visualize picking up an ax and swinging it down. No joke.
you mean how a guy like Alves seems to kick downwards to the leg?
Have your heel higher then your knee on the way down into the kick. Over rotate your hips to make this happen, especially if flexibility is an issue. ( using the leg as your target)
Two words... Mr. Perfect.
This kick comes from corkscrewing the plant foot and bringing your hip all the way around to your target. Bring the shin up and higher on launch and then drive it down just before impact.
Hit a bag with this technique a hundred times... Then you will always round house this way