You must kick as though you were drunk; the leg must be heavy and dead.
Swing your leg through the opponent - breaking their root.
Having a long slow arc robs the kick of any chance of surprise so you must employ them whilst grappling.
You must deliver them in a manner such that they do not alert the opponent's nervous system, employing the 3-legged kicking method.
The smooth, spontaneous movement must feel easy and relaxed, almost sleepy.
This is a 'gravity kick'.
Once the arc feels smooth and effortless, try to fa jing the kick without losing the heaviness.